I agree that because the pump was dispensing slowly, the back pressure device in the pump handle probably allowed you to fill your tank farther than normal. Filling your tank to the very top can cause problems, so it's best that, if this happens in the future, you stop a little early.
I'm not convinced that the problem was caused by all the pumps being in use. When I fill up at a pre-pay pump, and pre-pay for $20 (for example) the pump runs quickly up to $19.50 or so and then really slows down. I've filled up at stations where I was the only person on 6 islands, and the pump has run really slow, and I've filled up at crowded stations and have everything work just fine. So I assume the slow filling is a software glitch of some kind that keeps the pump on utra-slow mode.