I can't imagine doing that on purpose.
Educated guess - they were doing stall testing, and it got away from them. Although I don't know why they'd being doing low speed stall testing in a turn.
There is something we call a wind-up turn that's done to test the engine inlets at high AOA - basically set the test engine at high power, and start pulling a turn tighter until the aircraft stalls and falls out of the turn (holding altitude constant). But that appears to me to be a low speed stall, not a constant speed stall (which is what is done in a wind-up turn).