Glider fields by their nature invite more accidental contacts between people and aircraft than at power fields.
First of all there are more people about retrieving and putting gliders on the line plus a bunch of people hanging about waiting for their flight. With power fields the only people about are on the ramp or tiedowns.
Second, gliders don't make up much noise; so, they can sneak up from behind.
Remember also that Joe Student or hotshot competition pilot may have gotten low and is squeezing it in over the trees on one of the other runways who will not be spotted until he is on the ground. (Much better to pick out a reasonable field with sufficient height to fly a decent circuit).