If a player thinks he might get cut, then he may decide to leave on his own instead of leaving the decision in someone else's hands.

It is no different when 2 companies merge or 2 departments merge in a business. You will see some employees chose to leave for another job because they see the writing on the wall instead of waiting for their manager to tell them they have been fired/let go due to staffing cuts/budget cuts.

As far whether the coaching staff should talk to the recruits leaving on their own for whatever reason, it is still a numbers game. It is very likely these coaches don't want to convince these players to stay because their decision just made their jobs a lot easier because now they don't have to look for another player to cut to get back down to the scholarship cap. Plus, some of these players may not be missed because there was plenty of depth at that position. If the numbers get below the 85, then that will allow the HC to look in the portal for players at positions that may need more depth for the upcoming season.