This would be pretty frightening - especially considering the speed a cyclist could be going during a race. 

While we were debating what caused a Shimano trispoke wheel to explode at the Tirreno-Adriatico, it turns out 2 more failed off camera. Geraint Thomas said so on Twitter .
Shimano is investigating and released this statement to multiple news outlets, including Bike Radar.

“PRO is continuing its investigation into the issue we saw with Team Sky at the team time trial of Tirreno-Adriatico. We are continuing to look closely into all factors that could cause the incident.

During production the three-spoke wheel passed PRO’s extremely high internal quality control and ISO/UCI standards. PRO’s three-spoke wheel was introduced in 2014 and has a flawless record, achieving countless time trial victories since, including BMC’s team time trial win in the same stage.

In the meantime we wish Gianni Moscon the best of luck in the remaining stages of Tirreno-Adriatico.”

Bike Hugger Full Article: http://www.bikehugger.com/posts/shimanos-exploding-trispokes/