Were these guardsmen volunteers?

If you're talking about the Palace Alert program to use F-102s in Vietnam, I don't know. It probably was a volunteer program, at least for the Reservists and National Guardsmen. During wartime, there are a lot of things that happen where servicepeople don't get to choose whether they want to go or not. They may well have been activated and sent overseas like so many of the Guard and Reserves have been today.

What didn't Bush also volunteer for this duty?

He did volunteer but didn't have the required 500 hours of F-102 time. Most of the time, a fighter pilot gets about 250 flying hours a year when stateside, so you're talking about roughly 2 times of time in the plane. Also, by 1970, Palace Alert was winding down.

Why did he disappear?

Why did who disappear? If you're talking about Bush leaving the Texas Air National Guard, that was addressed in the earlier linked article. By 1972, the US had scaled down operations substancially in Vietnam. There were more active duty pilots than cockpits available stateside, so they had first dibs on the flying assignments.

Was he AWOL?

Not according to this man or this man. They remember seeing Bush at the Alabama Air National Guard meetings.

Now, did Kerry spend Christmas in Cambodia as he has claimed so many times, or is he acting out some Walter Mitty fantasy? Does any of this matter about either man? Why isn't Kerry talking about his 19 year record in the Senate, unless it's because he really has nothing to show for all those years.