One may learn much from the way things break.
 

This rim has blown. The sidewalls have been worn through by the brake blocks, weakening the rim to the point where pumping up the tyre has created enough force to split the rim. Once this starts, it gets faster as the inner tube hernias out before bursting.
There is usually a bit of warning before this happens; the rim will appear concave, wear indicators will appear and the man in the bike shop won't pump up your tyres as the rim will blow. Just before it happens there may be a juddering feedback through the brake lever.
If a rim blows on the front wheel while you are riding, you will crash and probably die. Get it fixed.
 
This is how to ruin your rim the quick way.

 
Wear away the rim with the pad and then its holder.

 

Then pay for them to be renewed. The customer is suitably ashamed and tells us he will check his brakes regularly from now on.