liquidations will typically begin at approximately 9:40 AM ET

http://ibkb.interactivebrokers.com/node/1718#What_can_I_do_to_prevent_the_assignment_of_a_short_option_

What happens if I am unable to meet the margin requirement on a stock delivery resulting from an option exercise or assignment?
If an option exercise or assignment results in the delivery of a long or short stock position and the account holder does not maintain sufficient equity to meet the ensuing margin requirement, IB will act to liquidate positions to restore margin compliance. While IB retains the right to liquidate at any time in such situations, liquidations involving U.S. security positions will typically begin at approximately 9:40 AM ET as of the business day following expiration.

I fixed the margin issue myself before 9:40am.