Looking at the youtube video of the radar with the radio sound that someone posted below, the Cirrus is cleared to runway 17R but he turns to runway 17L.
There are 3 possibilities :
1- he mistakes one runway for the other visually,
2- he wrongly thinks he is cleared for the 17L runway
3- or he makes a too wide turn going for the 17R, intruding the 17L area.

The cirrus is the one that makes the mistake.

The 17 runways are quite close laterally (700’) , it may be either way a bad maneuver (overshooting) or chosing the wrong runway.

https://es.flightaware.com/resources/airport/APA/APD/AIRPORT...

The metro was not expecting another traffic in approach for his runway (I understand that they were in different frequencies with different controllers).

During the approach the upper-right side relative angle position in the window of the metro, makes the cirrus hard to spot. I guess he didn’t see the cirrus at all or just barely before the crash.

The cirrus is looking at the runway to his right and the other traffic probably the whole time, the metro is in front of him, so he doesn’t see the Metro till he is on top of him.

Usually with parallel runways, traffics are kept at different altitudes till they are aligned with their runways. This way if they make a mistake, they are separated by 1000’ vertically with the airplane flying parallel.

In this case the cirrus was cleared to visual approach and informed of the cessna he had to follow first. Once he says he has the cessna in sight, he is cleared to visual approach following the cessna. In the same comunication he is informed of the metro flying to the other runway and he replies traffic in sight again.

My guess is that he either has the metro in sight at the beginning and then he forgets about it during the maneuver, or he gives traffic in sight two times.

Thinking that the second part of the message is for the same aircraft (the cessna) he doesn’t even recognize what the controller is telling him about the metro. This is possible if he is too busy flying the maneuver and not paying proper attention to the radio, he hears
“cleared for approach” and “traffic” but he mentally don’t really process the information the controller is giving him. A kind of sensory overload.

In airliners we have mandatory TCAS (traffic collision avoidance system) installed that shows you the near traffics in the screen and give you coordinated (between the traffics) automatic avoidance guidance and alarms( one traffic climbs and the other descends or keeps altitude).

In busy airports TCAS maneuver happen relatively often (a handfull of times a year) but nowadays is much harder to have a collision or a close call.

Also when two pilots are in the cockpit (like airliners) it’s easier that one is concentrated in flying and the other in the communications. It’s very common to correct and be corrected all the time during the flight.

It will be interesting to read the official report.

Edit: Kudos to the Metro pilot who was super calmed in the radio while declaring emergency and landing the plane. That is really difficult.

Edit 2: correcting the airport , KAPA (I talked about KDEN initially which has the same runways but with a bigger separation). This does make a difference regarding the mistake.

Thank you Denvercoder9 for the heads up.