That definitely looks like it was bumped from the outside by something solid. I cant see how that could happen from pressure on the seams.
what is that tank standing on? maybe one corner of the base is raised slightly and it has cause a massive build up of pressure in that area?
As long as it is not going through the glass anywhere you should be fine though. If it was me I would silicone another piece of glass over to help me sleep at night and just put a trim strip around the top of your stand to cover it.