There may be different things causing the behavior- anxiety, social discomfort, separation issues (especially if he is the only child) Something may be triggering this behavior specifically at school (possibly others bulling him, teaches not paying close attention to his needs/wants)
If you have not been told specifically by a teacher/staff at the day care what had initiated these behaviors, you may want to ask the to explicitly describe if something takes place before his behaviors start.
He may not be used to the routines at the day care and perhaps he is getting too tired and therefore acts out in frustration.
He is too young to be able to specifically rationalize his behavior. You may want to engage him in some pretend play at home with his toys (if you have some puppets or humanoid toys) and pretend that you're a peer at the day care. Ask what does he usually like to do/play with at the day care, do others share, does he have a good friend, etc. Then see how he interacts with the other humanoid toys. Tell him that you and him are just pretending it is a usual day at his day care and that you really want to see what they do and how he does it.
He may be also having low self esteem (depending on what events he is supposed to partake in at the day care) and if he senses that he's not performing as well gets angry and acts out instead. Behavioral is generally goal driven. His behavior may be attention seeking (even negative attention is some attention received)
You may also consult with a behavioral modification therapist who can use play therapy, narrative therapy and other hands on modalities to figure out and help him manage his anger.
A wonderful book to read to him-
A Volcano in My Tummy: Helping Children to Handle Anger by Eliane Whitehouse and Warwick Pudney (Paperback )