So two theories that come to mind right away - not sure how likely any of them are for your specific drone:
1) loose connection to one of the motors or similar - in the middle of the turn the connection to that motor or something, maybe gyro, breaks up for just a tiny fraction of time and the FC tries to compensate by throttling up all the remaining motors.
2) setting in betaflight, maybe related to crash recovery or something - the drone thinks it's in a crash/has just hit an object, and tries to recover - not sure what would cause that, maybe either faulty config in betaflight or a intermittently loose connector to the gyro or something.
(The loose connections suggested doesn't really have to be wires, could just be a loose solder joint inside a component/motor)