Almost impossible to say although noise on the MCLR pin or supply lines are likely causes. There's a register, I believe it's called RCON (check the datasheet), which you can check to see what caused the reset. There will be various bits set depending on if it was a brown-out reset etc - that might help.

Make sure you have sufficient decoupling as close to the supply pins as possible and make sure you coonect ALL the supply pins - not just one pair.

/Henrik.