there are a lot of theories. the most prevalent one is that ultra-high par and especially high UV can trigger bouncing, as a sort of defense mechanism. However that hasn't really been tested, and if it has it's been done by vendors and they've been selling the shrooms they make lol
Just keep in mind that shrooms like the OG bounce and Sunkist were collected from amazingly shallow water (deep enough to wade in at low tide, if my memory serves me right) and thus would be exposed to this high light at all times. But who knows, it could be anything. Some strains of "bounced" shrooms will lose their bounces after a while (like Juggernaut and others), which tells me that they aren't bounced in the same way.