I sometimes think that parents stay when their child cries because they feel responsible and would feel bad leaving the caregiver to deal with it. Maybe if you send an email out to mom identifiying that XXX is having a hard time lately and you understand it and are ok with it. It seems to make him more upset the more mom lingers so please come in, say goodbyes, tell him/her you'll be back later and out the door!

I've had kids that were criers at first, got better and then suddenly out of no where started up again. Who knows why... but mom's tend to hang on longer as they think there's a problem. It's fairly normal from my experiance.