Just a phase? Meltdowns near adults

Ok this started about a month ago when the maintenance guy came to work on something in the apt... happened again a week later when he came back so we figured he was just having an issue with the one guy or because someone came in the apartment unexpectedly... but nope.. he has been doing this with ANYONE who comes in the apartment, even the grandparents.  He had a meltdown when they showed up which lasted about 10 minutes and had him staying in his room till he got his usual happy self back, then he was fine...

It happened agin last week when we visited the inlaws at their house.. again 10 minutes later back tpo his usual self.

The other maintenance guy just came over and again .. meltdown but that only lasted about 3 minutes... and I mean full on tears and hysterics.. not just shy and hiding.

He is fine in places like Walmart which is crowded with people and adults ..when he goes to the park he is fine with other kids ..till the parents come close then he gets a bit shy and clingy...

This lil guy  has always been mr social butterfly  with everyone ..and he has known all of these people  practically  since the day he was born... any  ideas? just  a phase?  

as a side note ..Nothing bad has happened with any adults and he isn't in daycare, or left with anyone but me or his dad.. so I know nothing happened to scare him when we weren't around.