same thing happened to me today. The only position i have in my account is es reverse calender spread. I calculated exactly how much loss i will take from theta and vega and had a $5000 excess liquidity.
Then when i come home today from work, it freaking liquidated 2 of my contracts at 4:30PM!!!!! The contracts at the time had a 30pt bid/ask spread, and the fucking thing went and hit one of my legs at market price, and of course all went to hell as the combo is no longer balanced and it then hit my other leg at market too.
Now my excess liquidity is flipping back and forth between $100 and $1500 and i am getting spammed with margin violation emails. I am going to up at 7:30 tomorrow and trying to close out more of my positions to bring back more excess liquidity, hopefully it wont go and start liquidating my contracts in the middle of night.
I really dont know what's happening, whether it was my stupidity or IB's margin calculation changed, or a bug. Reverse calendar spread has well defined risk and today is even a great day as vix crashed and the position is short vega. I still have no idea how it went from +$5000 excess to liquidating my position at fire sale when the market is moving in my favor!!
Of course calling IB doesnt do crap, this is one of the downsides you have to accept with IB for able to use their wide range of products. This compared to optionsxpress, where i had a margin warning a long time ago from a spike and the vp of their margin monitor team was email/on the phone with me within 30 mins and we worked out a method together that made sense based on the combos i had.
I loss about $3k today from the liquidation, the worst part is i still dont know how my excess went from +5000 to negative.