Tempered glass is stronger than annealed glass, but can break. The reason it is used is because when it breaks, it shatters into thousands of small, relatively harmless pieces instead of a few large, dangerous shards. The problem with tempered glass is that even a small impact or scratch can cause it to break. And sometimes it breaks just because it wants to. The main rule of thumb when using glass is to use it in a way which is not likely to cause it to fail, and use it in a way that if it fails the damage is minimal. Sounds like you are in danger of violating the second part of the rule.