Photos of the nests might help, but if they have been laying for a while in the nests and they’ve switched that’s probably not the issue. How old they are, how many nests you have, the number of hens or pullets you have, how long they have been laying, if this is an established layer or a pullet’s first eggs, all these things could have a bearing on it. With chicken behaviors it’s sometimes difficult to determine what is actually going on or why.
Something like this is pretty common when they first start to lay. The vast majority of pullets lay in the nests where the others are laying from the start, but occasionally I have one of them do exactly what you describe. I had a pullet just start laying do that about a week ago.
I’ve also had hens that were established layers and laying in the nests stop doing that and “hide” a nest somewhere else. That might be a corner of the coop, somewhere else in the coop like under my nests (they are pretty close to the coop floor), or outside somewhere. I don’t know why they do that, probably different reasons, but it happens. With living animals you don’t get any guarantees as far as behaviors, about anything can happen and you don’t always get an explanation.
I built a couple of nests that I can lock a hen in if I want to. That’s actually come in handy for different things. When I have one laying where I don’t want her to, when I catch her on that nest I remove her and lock her in a real nest until she lays her egg. I’m not real gentle when I do that. Sometimes I can just pick them up but often I lock the coop door and use a fishing net to catch her. It’s OK with me if she thinks that is not a real safe place to lay eggs.
Usually it takes them about a half hour to lay that egg, but I’ve had some take three hours. Usually I only have to do this once but I’ve had a few that it takes a repeat performance.
With all that said, it sounds like a pullet started a nest there and some of the others decided yeah, that looks like a good place for a nest. That’s not unusual behavior either. They like to lay where others are laying.
Why is there poop in the nests? Are they sleeping in there? If so that’s a problem I think you need to solve. Photos of your nests and roosts and the age of your chickens might help us come up with a solution that fits your situation. I don’t know if this has a bearing on the other problem, but it might.