There seems to be a glitch that makes all blobs die at once for no reason. I have no idea what causes it. It definitely wasn't a load of explosions at once last time I saw it happen.
It didn't look like an enemy reached the center tile either. It was pretty early in the game (I don't remember which level, around 30 or so) and I had a full field of blobs. They suddenly all died at once and there barely was anything going on.
