Usually such students are unaware of the problem and are simply outgoing by nature. If the student is aware of t he problem, usually he feels he has not had his point fully appreciated, or feels some need to compete with his classmates. In all three cases, just recognizing the student this way often reduces the student's need to dominate conversations. A Student has No Interest in Speaking. Some students feel that they learn better by listening than by talking.
Some students feel that speaking and helping others requires too much effort.
Talk to the withdrawn student about helping others. Explain how you think they could be very helpful to the learning of everyone else. Most likely the student will feel flattered for being taken aside like this and will want to contribute a t least a little more. It is quite easy for a group of students to all mistakenly agree that when they get zero over zero they can cancel to get one. Or to all misread what it means for a function to be continuous.