Vehicle bearings keep breaking?

On my 1998 Ford expedition the front driver's side wheel bearings keep breaking. I have replaced them twice to no avail what could cause this? The only thing I can think of is that the dust cap is loose and grease is leaking out but I'm not sure if this would cause bearing failure.

1998 Ford Expedition XTL
2wd
5.4L V8

Read more: Vehicle bearings keep breaking?