There is a lot of good things about IB, but their auto liquidation system is so customer unfriendly I would never recommend them for anything other than buying options. Your situation illustrates this perfectly. There was no greater risk when you were assigned, as you had options against the position, in fact there is less. This is all about margin from your assigned stock. What a firm "should" do is allow you to meet the margin call or allow you to close the position in the most efficient way possible, which here would be entering and order as a spread to sell your stock and your puts. If you don't do this they should do it this way.
For those people who say "I'm glad they auto liquidate, it protects me" that's BS. If someone is short a put spread and are assigned on their short puts, they now have a less risky position. There is no urgent need to immediately liquidate everything (as opposed to being short puts when the market is falling, this is when their is an urgent need to liquidate immediately). This creates more risk and losses, which is more dangerous. Suppose the trader has a butterfly as part of their position, auto liquidate sells each leg out separately for a huge loss.