Infestation

Cooper (Chris Marquette) is a slacker and he works as a telemarketer. On a Friday, he turns up late for work and is promptly fired by his boss, Maureen (Deborah Geffner). He is fired for coming late, driving customers away and for fooling around in office. Shortly a sound is heard and Cooper faints with pained expressions on his face.
Cooper wakes up a few days later, he find himself nauseous, weak and covered in webbing (cocooned) in his office. As Cooper struggles out of the cocoon, he is attacked by a big bug. He fights the bug away and helps Maureen out of the cocoon. Maureen remembers that her daughter, Sara (Brooke Nevin) is waiting for her outside the office. Cooper and Maureen go out to the office to check on Sara, who is unconscious and trapped inside a cocoon. As Cooper is helping Sara out of the cocoon, Maureen is attacked and captured by a flying bug. Other bugs surround them and they escape to a restaurant.
There they revive Cindy (Kinsey Packard), Hugo (E. Quincy Sloan), Hugo's father Albert (Wesley Thompson), Leechee (Linda Park), Roger (Bru Muller), PJ (Vlado Mihailov) and a Cop (Todd Jenson). PJ and Roger decide to escape in Roger's truck against the advice of others. They are attacked by the bugs and in the chaos PJ, Roger and the cop are killed. Albert is stung by a bug, Cooper saves Cindy from coming under Roger's truck and rest of the group decide to stay indoor.
Leechee realizes that the bugs are blind and they use sound to track their victims. They use a radio to trap a bug and milk it to identify the contents of its venom. Leechee analyses the venom and realizes that it contains sedatives and some proteins. They decide to go to the nearest Military airbase at Gibson the next day and decide to camp for the night in the same building. They also see the bug hive, which is emitting red colored gas. Leechee decides to stay back to help revive more people and to study the bugs.
The rest of the group sets off and goes to Hugo's and Albert's house first. Albert's wife seems to have died of natural causes. They next go to Cindy's brother, Chad's (Mike Straub) house. Cindy finds her sister-in-law Susan (Daniela Tonova) cocooned and revive her. They also find Chad, but he has morphed into a hybrid, human body with legs like bugs. Chad is collecting cocooned humans and he attacks the group, he kills Susan, knocks out Sara, impales Cooper when he tries to save Cindy and is finally killed by Hugo and Albert.
They camp for the night at Chad's house and Sara patches up Cooper. Cindy wants Cooper to ditch the group and come with her but Cooper is in love with Sara and he turns down the offer. The next day they decide to go to Cooper's dad, Ethan's (Ray Wise) house. Ethan is a former Military personnel who has weapons and a bunker at home. Sara informs Cooper that she intends to go to the bug hive to find her mother. Cooper tries to convince her to come with him to a Military airbase but she doesn't agree. They come across few flying bugs and hide under the bridge. Cindy sees Cooper and Sara holding hands, this makes her angry and she shouts to attract the bugs towards the group. Albert shoots Cindy to save the group but Sara is captured by the bugs. Finally the group reaches Ethan's house and Albert turns into an hybrid. Hugo shoots his father to save others.
Cooper tries to convince Ethan to help him save Sara but when he fails he leaves Ethan and Hugo behind. Cooper is captured by another group and they lock him up in prison. There he meets a Puerto Rican Man, who tells him at he was part of Leechee group and that the others died in an explosion caused by the inflammable red colored gas released by the bugs. Hugo and Ethan are captured by the same group. It is revealed that Ethan has been stung and the Puerto Rican Man turns into an hybrid. Ethan uses the distraction to kill the members of the other group.
Meanwhile, Sara wakes up in the bug hive and comes across the queen consuming cocooned humans. Ethan, Hugo and Cooper reach the hive. Ethan hand-cuffs Cooper to a tree and hits Hugo in the head. He leaves the keys so that Cooper and Hugo can escape. Ethan enters the hive alone to destroy it and he comes across Sara. Sara offers to take Ethan to the queen. Ethan and Sara are attacked by the queen. Hugo and Cooper enter the bug hive, arriving in time to help Sara and Ethan. Hugo throws the explosives into queens mouth. Ethan asks Sara, Cooper and Hugo to escape from the bug hive while he triggers the explosive. But Ethan turns into a hybrid, Copper takes the remote from his father and triggers the explosive.
The film ends with Cooper, Sara and Hugo turning toward the sound of a large rumbling. The source of this rumbling is not specified as the film then turns to black in a cliffhanger. The rumbling is suggested to be either the insects returning or the Army finally arriving to attack the alien infestation.

Question: What does Cindy see that makes her angry?
Answer: Cooper and Sara holding hands