I've seen this. It seems to be a small glitch in the game.

When there is already a peep resting in the hammock and another decides they want to go there as well, they will walk to the hammock then shake their head when they realise someone has beaten them to it. You'll often see similar behaviour when a bathroom sink or computer is already occupied. In some cases the other peep needn't actually be there, just on their way; they've already "booked" their place.

The odd thing is that whereas the game will display "Brushing teeth" or whatever before the peep fails to get there in time, it shows "Nothing" in the case of the hammock.