It could be for various reasons. Personal issues or emergencies that require them to be away, a loss in interest in coven affairs, realizing that leading a coven is more responsibility than they originally thought, and so on. In any case, I feel that they should place someone else in their position to assist the coven and its members.
My coven had an issue like this a few months ago. One of our previous leaders was absent for quite some time and then returned once their position had been revoked. They then left yet again and, as far as I know, they haven't been back to the site since. Thankfully, the issue was resolved and all is now well.
I agree with you. Unless their is an emergency of some kind that prevents the leader(s) from logging in, there is no excuse for accepting or being granted the position and then leaving shortly after. I also feel that they should be banned from leading or applying to lead covens in the future. If they aren't there to lead their current coven and assist their members, what's to say that they will be there to do so in the future.