I think it would only break if hit/struck or if too much pressure was put on the glass.
Shattering due to a hit wouldn't take too much force if the striking item was small, narrow, or sharp. Is there a small tear in the cover? Maybe it was a stray bullet from a drive by...
As far as the too much pressure scenario goes, I assume your cart is like most and the bottom support rests against the glass. If something or someone pressed down - for example, leaned heavily on the front edge of the top - it could transfer enough force to the glass to break it.
Sorry about the breakage. Let us other hardtop owners know how much it costs to put right. I have a ceiling hoist for my top... I chose that approach to minimize the chance of damage.