What should have happened is your Long Options position taken on Friday acted as hedge for one off your short positions in strangle, giving you margin benefit.
When you squared-off your Long Options position, hedge was removed as result of that margin requirement shot up and since you didn’t bring in required margins after getting margin call, your one short position resulted in auto square-off.
Could you also help me with one follow up question: On Wednesday, you’re answer makes sense exiting long call triggered auto squared-off of 50% of short position (in one leg of short-strangle). But on Thursday, another 10% on the same leg was auto squared-off (No buy/sell from my side). What could be the reason for that?