Dear Avram,
First of all, I wasn’t in that yeshiva.
1) Some people in chinuch dropped their own nuanced hands off approach when it came to yeshiva bachurim and their phones. The fall out -even today- is tremendous. The yeshiva staff did not see anything from the angle of the student body. They were continually at war with some talmidim.
2) The boys that were not committing to permanently giving up their phone, had no one to talk to. Half the yeshiva was put on a pedestal for not having a phone even though it wasn’t a sacrifice. And the reaction to this group was that if you were interested being better off, you would join them. So the yeshiva wrote them off as not intersted and overlooked them for everything.
3) Bachurim had to get rid of their phones within a week. They had to figure out if they should cancel or not. They had to put all their info somewhere else. Some had to figure out new travel arrangements. It disrupted a lot of the routine. And it encouraged face to face encounters that were not desirable.