Why would our golf cart batteries start bubbling after about one half hour of charging?
Hi, Maynard! Probably because your charger is delivering too much current.
Normally, flooded lead acid batteries will boil a bit when they're charging, but toward the end of the charge. That's the bubbling that's going on, the battery acid is boiling.
It's not unusual when charging these batteries. They boil, they vent a little (hydrogen) gas. If you overfill them with water, they'll even boil over, accompanied by the delightful odor of rotten eggs.
But boiling 30 minutes into the charge, that sounds like a malfunctioning charger, or a voltage mismatch. You're using the right voltage charger for your pack, right?