Suddenly the project I was working on sounded radically different on playback. I was baffled for a while until I eventually discovered that somehow all the Prochannel Compressors in every bus had become active. Obviously this wasn't immediately evident as the Prochannel is not visible by default. Any ideas how this could happen?

yes, possibly by accident you used quick groups.
holding down CNTRL and clicking on 1 PC Bus compressor.
 
if nothing is selected and you hold CNTRL and click something, move a slider, it moves every slider, button.
 
just try it, hold down Control without selecting any tracks or buses, then click the any PC bus compressor.

Thanks, I'll check that out. I've never used Quick Groups before (except maybe unwittingly if your suggestion is correct).

Yes, that was it.
 
Quick Group seems like a useful facility that wasn't really on my radar until now. I'll be making use of it in future.
 

 
I figure that was it.
 
here is a small list about quick group rules:
Quick Groups only affect tracks of the same type (Audio or MIDI).           
For instrument tracks, the relevant controls is treated as belonging to an Audio or MIDI track.              
Quick Groups do not work between tracks and buses.              
Quick Groups apply a relative adjustment to each control.              
If you adjust a control in a selected track, only selected tracks of the same type are affected.             
If you adjust a control in a non-selected track, all tracks are affected.
 
I look at "grouping" as 3 different modes (even though they are only 2 = Quick + Control groups)
 
drag and select multiple tracks- (Example: drag and select the guitars on tracks 5 thru 9)
so you drag the mouse in console view or track view to highlight all these tracks...
now hold CNTRL and move a pan knob. you'll see tracks 5 thru 9 pan knobs move all at once.
 
Select none-don't select anything. hold CNTRL and move a fader, knob, button.
they all move. this is good for lowering every fader in a project that is running too hot.
 
Control groups- These are the groups that use colors. I still use control groups often because of their depth.
one can set them up in 3 ways. relative, absolute or custom.
these are useful if you want one control(guitar fader) to trigger another control (a different guitar fader)
so raising one fader will intern lower the other fader.
 
Anyway, Glad it's sorted and happy 4th weekend.
 
 

chuckebaby
Control groups- These are the groups that use colors. I still use control groups often because of their depth.
one can set them up in 3 ways. relative, absolute or custom.

One of my favorite uses of Control Groups is A/B comparisons. Enable Mute on the A track, don't enable Mute on the B track, then group them. Now enabling mute on one track disables it on the other track.