It is not out of the realm of possibilities that the heater caused this. If it did explode I am sure it started to expand first (since its plastic), which would cause the heater to pop out of the suction cup holders. If that is the case, and the heater exploded, it could have most certainly hit the POI as shown in a previous post. As to the above poster, anything hitting glass hard enough will break it, even plastic. So the placement of the suctions cups is actually irrelevant to anything IMO.
Now, since it is plastic and had to expand before it "exploded" that could have caused an increase in pressure within the heater, causing the explosion for lack of a better word, which would cause the heater to smash into the glass.
I fully support the OP's story for the simple fact that it is obviously not the first time this has happened to someone. If this was the one and only case, I would be skeptical too. But, this even happened to another poster on this thread, down to the exploding portion.