I've had the same issue before with the weight and balance of the hold cargo. Same situation, after pushback we had to taxi back on to stand. I guess the loaders have to do some paperwork for ground records and realised something was wrong.

The passport checks happen quite often. I think these tend to be targeted at flights from countries where they expect to receive asylum seekers or potential illegal immigrants. I guess they are trying to surprise them when they least expect it!