You don't say how old the child is. There are periods that for some reason kids go through an adjustment period and yes they can start to cry at dropoff or at the slightest thing acting like they have anever experienced it even though you haven't changed the process. 9monhts, 12 months, 15 months, 18 months,.....note a pattern here. It somewhat has to do with the child learning new skills no matter how minor they are. The move to being able to walk is the biggest one. The other point is where they are going through a maturing growth spurt and language explodes - ie they become more aware of the world around them. Teething can also through everything off and especially if he has been touchy at home and is getting carried or otherwise being reinforced for the behaviour.

Be sure that in caring for the babies you don't get so busy that you leave the older one to his own devices for long periods of time. Trying to include him more in what is going on if he will - sitting with books or toys near you while you feed a baby. Make times during naptime for you and him to play a special game which I assume you are already doing.