Jack, I put a note on one of the MilAir posts a couple of days ago about that. The 123.75 is an alternate Tower freq and they've been switching to it for periods of time and then back to the normal 119.4 freq. I have no idea what's up unless they are doing maintenance on the radios. A lot of the time, the Tower will simulcast on 123.75 while using 119.4 as a primary. I haven't heard an alternate UHF Tower freq.
Ground Control will sometimes switch to their alternate too...120.2 rather than the normal 121.9. Both 120.2 and 123.75 are shown in the database.
Alan