Why does my daughter cry when dropping off at Montessori?

She has been attending daycare/Montessori since she was 12 months old and stopped crying at drop-off after the first month or so. In fact, she was very happy and excited at drop-off for the past few months. She is now 16 months old and has started crying at drop-off again. There has been no change in the teachers at the Montessori. She attends 3 days/week and is happy while she is there. She stops crying within minutes of me leaving and is happily engaged in activity during the day. Why has she reverted back to crying at drop-off after doing so well the past few months?