I was driving ( overdriving ) my Avalon 747 and it started to smoke. I
quickly unplugged and opened it up and everything seemed okay.

After about 20 minutes, I plugged it back in and it seems to be
functioning fine

Just to clarify, the signal chain is as follows.

Guitar Boss MT2 Allen & Heath Mixer Insert BBE Sonic Max
Avalon VT 747

Most levels were normal (-3dB) except for the 747, which was cranked
to 11 in all respects.

The guitar tone was incredible and I was rocking out until smoke came
billowing out.

Are there any ideas as to what might have happened?

After I opened it up, I could find no trace of any components burned
or charred. Everything looked clean.

Again, everything seems to be working fine now.