They were testing the escape rocket system on the recovered test crew capsule.
No comments from Musk or SpaceX apparently.

SpaceX Suffers an Accident During Ground Test of Crew Dragon Astronaut Capsule https://t.co/if67GeNhq0

— Military.com (@Militarydotcom) April 22, 2019

SpaceX EXPLOSION: Watch SHOCKING leaked footage of the Crew Dragon capsule in FLAMEShttps://t.co/2y1R8Lxj6G pic.twitter.com/PMZmyPTXpk

— Daily Express (@Daily_Express) April 22, 2019