Anyone with kids- can you answer this?

Abby R. • Wife, Mom to Mickey and Phoebe 💙💜
I've been babysitting for a family for about 2 months this now. I have been doing this since I was 15 and I'm great with kids. Their son is 3.5 yo and the past week or so, when his mom is about to walk out the door he HISTARICALLY cries and tells her he doesn't want her to go. As soon, and I mean AS SOON as she shuts the door behind her, the little shit stops crying and wants to play😂😫 I've never had this problem with any other family before. Why is he doing this? 
I don't want the mom to get the wrong idea because he's doing this (although when she gets home he always hugs and kisses me goodbye and says see you next time). 
Opinions?