I would say it's mice, they go by the smell of the peas germinating. If you water a weak solution of Armillatox over the ground after sowing your peas, it should keep them away.

or start them off in pots and then transplant with as little root disturbance as possible.

Place a a piece of wire mesh over them also.

Harry Dodson (Victorian Kitchen Garden) put holly sprigs over the peas before covering them.