We all know a student is expelled because he committed something extremely bad. But how is extremely bad? Must the student commit a crime or something? Unfortunately, university policies do not give details of that. (In America and Canada)
As an undergraduate, I served on the disciplinary hearing board for a large private secular university in the United States. I saw the full spectrum of cases that came before the board, from minor infractions to severe violations of university policy (and the law).
In my experience, the university was reluctant to fully expel someone. Minor infractions typically resulted in some form of mandatory training and/or disciplinary probation. Repeated minor infractions might result in a short suspension. For most severe policy violations (e.g., assault), the university preferred to hand out lengthy (1-2 year) suspensions rather than permanently expelling the student. Obviously, I'm sure that many students chose not to return after this setback.
The exception to this was sexual assault cases, which provoked a more unequivocal response from the university. Expulsion was certainly on the table for cases where a student was determined to have committed sexual assault. At the very least, the perpetrator would be suspended as long as the victim remained on campus, to ensure that the victim did not have to interact with the perpetrator. Inevitably, Title IX requirements and liability concerns shape both the hearing process and the outcome for sexual assault cases. This type of scrutiny was generally absent from other cases.
I can think of one other case where the hearing board recommended expulsion for a student. In that case, the student dealt tens of thousands of dollars of hard drugs to an undercover police officer over many months. The student was arrested and the university subsequently began an internal hearing process.
The standards and process may differ for other types of universities, but generally I got the sense that my experience was representative of other large secular universities in the United States.