Why do kids act out the day after transition?

Every single time I drop them back at their mom's house, they're amazing that whole evening and next morning. But by day 2, they act like monsters. Full meltdowns over nothing, rude, disrespectful. Is this normal? Are they processing the transition still? Should I mention it to their mom or will she just think I'm doing something wrong on my week? They're 6, 9, and 11. Mom hasn't mentioned anything about it but I feel like she's not being totally honest with me anyway.