Originally Posted by 3'senough
Sounds like backwash. Do you have a clear elbow on the hose to confirm that your tank is draining fully? Could have had a block when you thought it was fully drained and while the added water was more than what was going out your tank was filling higher and eventually got to the bowl. If your tank fills higher than the vent stack air will come up through the bowl and create a whoosing sound if the ball valve is closed. Don't ask how I know this...
Sounds like you avoided a huge cleanup by a few mins.
Thanks, this had me really puzzled. But I can see with a blockage, even partial, this could happen. Once I read your post I remembered going to the hose and feeling it and thinking, there doesn't sound like much water flowing. (duh)
For it only to fill the toilet bowl, I'm thinking the back pressure probably got sufficient at that point to free the blockage. You are right, I was moments away from disaster. From now on my wife will be monitoring the bathroom while the flush is on and I shall not go off wondering around.