How does this happen while riding??

[veech_imp]

This can easily happen from a couple things:

!) the set screw was too tight, which may have caused a stress crack that eventually got worse through the bike vibrations and broke

2) aftermarket lenses, which are typically a harder, more brittle formulation that the more plyable (and shock absorbant) OEM lens

I've dealt with this for years with Harley aftermarket lenses, to the point where I finally got sick of it and went back to OEM lens and only tightened the screw just enough to be snug and used Locktight. Stopped breaking any more lenses...