Apparently a fix will be out tomorrow for this gameplay effecting annoyance. If you haven't seen it This Video shows how bad it is for some people.
I myself have had a slight case of this, but nothing like what is shown in that video. It happens because textures are still being loaded into memory as you play. This is a technique is used in Lineage 2 and other games so that you don't have to wait for levels to load, but HL2 probably just uses it to cut load times. So expect load times to increase after the fix is release (probably tomorrow).