if the rest of the pack feels their is nothing else he/she can offer the pack. remember the alpha is in that position because of his knowledge and experience.if the pack feels the alpha's knowledge and experience is no longer enough to lead the pack then he can be challenged and possibly kicked out of the pack.
now sometimes even if the alpha is beaten he can still remain in the pack but that dosen't mean he will be in the highest position.
Yes, if they were challenged for their position and lost.
If you feel like they are a threat to you or if they are corrupting the pack, then yes
Generally the alpha only when becomes to old and is challenged , other members yes .
Yes I've usually young males who may become a threat to his leadership .