Ryan likes his shield generator, he does. Smiling just a bit, he takes advantage of the break to switch out weapons and pick up extra ammo and explosives.

Which is about when it occurs to him that the Combine might decide grenades could take him out. They won't, he knows that much already, but they will send him flying.

So he might as well send them flying first. Anyway, its a good way to determine precisely what these grenades do.