They could be standing there forever I guess but as soon as they are spoken to they suddenly realize they′re in a hurry!
They weren't just standing there. They were relieving themselves by the side of the road. It's one of the advanced features of Bethesda's Radiant AI. They are programmed to express intense ambarrassment when they are caught in the act. They pretend that they have something
real important they have to do and they get back on their horses as if nothing unusual was happening.
If you follow them, you find that very soon they will get off their horse again when they think nobody is watching. They will stand for about a minute, whistling to themselves, and then they will get back onto their horse and continue with whatever it was they were doing before.