Here's my theory; 1) guests move randomly, but not randomly enough to remove themselves when the reach a "dead end". 2) There are a limited number of guest brains, and therefore after a certain number of guests, some are using the same brain - so both guest #4 and guest #236 are thinking "I need to eat, I need to pee" at the same time.
Bear with me here, it's a weird theory . . .
3) Guests can get stuck in dead ends. If you put a discovery kiosk on a 2-tile path, guests can't get around it. Try walking between some objects in first person mode and you'll see that although it appears that you can go between objects, you actually have to take the long way around.
4) If guests share a brain, when guest #4 is stuck in a dead end, guest #236 is also unable to move.
I had a horrible problem with this in the meerkat zoo. I finally realized that I had placed my discovery kiosk at the entrance where there is a T in the path that is not quite wide enough for guests to move around. Guests were just standing around all over the zoo, saying there was nothing to do. Once I moved the kiosk, the log jam broke up *all over the zoo*.
It's also happened where I had a 2 tile path with a gift cart across from a bench or donation box. Looks big enough to get through, but guests get stuck - invisibily.
So - - go hunt for objects that just might be weirdly blocking progress somewhere. Good luck and I hope this helps.