Children sometimes stiffen up when they're having a bowel movement, especially if the stool is hard.
Another theory is that your child is simply stiffening because he's excited or frustrated. He may also be discovering new ways to use his muscles. Some babies stiffen up when they're doing something they'd rather not, such as getting a diaper change or being put into their snow suit. If your baby shakes or his eyes wander and get sleepy when he stiffens up, consult his pediatrician to rule out any neurological problems. But if he's responsive and interacts with you during these rigid episodes, he's probably just fine.