This is a volatile topic. What is saddest is that the Gedolei Yisroel of recent generations have provided guidance on the subject, and the current practice is to ignore that completely.
The expected arguments include the mismatch of a talmid to a yeshiva, and that expulsion rectifies this. We will also hear a position that some behavior is that serious that expulsion is the only fitting consequence.
There are other reasons besides behavior that have prompted yeshivos to throw out talmidim. One of them is the non-payment of tuition. That subject has yielded much rage, and gedolim have spoken up on the matter. We all know that running a yeshiva is expensive, and that most yeshivos are running at a deficit. How expulsion solves the problem is still difficult to explain.
The expulsion issue today is even more serious than it was decades ago. The street is an extremely dangerous place. And getting accepted after being thrown out is harder than scaling a cliff. But I do not run a yeshiva, and cannot address every single aspect to the issue.