If there is a particular area they choose all the time, maybe you could do something like placing some plastic mesh into the grass. I don't know how you'd do it, but work it in flush with the ground so that the grass grows through it. That would keep them from digging and you could still mow over it. Or you could erect some kind of decorative fence around the area you're wanting to protect -- dig down a few inches and lay a layer of brick or rocks below the ground level so they can't dig underneath the fence.
My guess would be it's because this spot has the right level of moisture and/or shading, so that the turtles think it's a safe place for the eggs.
Can you show us a picture of the area you're talking about?