Pirates like to come on in the late afternoon and evening, at least during the week, because that's when the DXers are listening. 6920 and up propagate well at those times. Also, the frequencies are typically ones that aren't likely to interfere with non-pirate stations. 6925 is just the latest such frequency; 6950, 6955, 7415, and others were in use at one time, but abandoned because of other users on those frequencies.