Typically chins dont shed like other animals like all at once in a season. They almost unnoticeably lose hair all year long very lightly. What you are noticing sounds like something called fur slip. This is when a chin gets startled and releases its hair from the root in a defense mechanism. It could be that he is frightened by something or possibly the way he is being grabbed or held.
Some chins just do it a lot and you cant do anything for them to ease their anxiety, others hardly ever do. It is not harmful or painful to them and will grow back.