They blew it up when the boosters failed to separate at about 10 miles. Better to blow up before orbit because debris would have scattered over the earth instead on f staying local. This launch was a huge success!
Partial postmortem..... Raptor exploded causing the APU which provides hydraulics to gimbal the engines blew.. total loss of engine control which caused the tumble ... failure to separate also due to holding pins being hydrophilic also.
from Nasa Space flight youtube feed
The on-screen altitude ticker said they were at 34 Kilometers. That's 21 miles high.