Short Call Assignment

I sold 10 call spreads on the 25th (expiring 26th) on GLD (374/375) for $150. GLD closed at $373.66 on the 26th (last Friday) at 4 pm, and my option positions still showed open at that time. Come to find out this morning that 9 of those contracts expired, but the 10th was exercised. This morning I had 100 GLD shares sold short in my account. Good news is that Gold was down today, and I ended up making $400 when I bought to close. However, I have questions:

- Why would my short calls be exercised if the stock ended below the strike price at 4 pm (and my positions showed open at that time)? The stock did cross $374 around 5 PM in after-hours trading.

- If there was a reason for the assignment, why only 100 shares and not 1000?

- Also, this is an IRA account (so no margin). Why would TOS allow me to hold a short position in a cash account?