What's up with this behavior at pick-up?

My daughter is 16 months old and has been going to daycare since she was a little over 11 months old. At first, she was always super happy to see me when I picked her up in the afternoon. She would run over to me with a bg smile and give me a hug. Over the last few weeks, she acts happy when she sees me but then she grabs whatever kid is nearest to her and either pushes, pinches, or hits them! It is the oddest thing and I can't figure it out. Her teachers say she isn't normally aggressive, although it is typical for kids this age to display these behaviors sometimes. She is always really happy in the mornings when I drop her off and gives her teacher a big hug so it seems like she is happy there for the most part. Anyone else had this happen to their child?