Weird, fascinating phenomenon on Yes Fragile LP

I'm totally intrigued. Maybe you can help me figure this out.

On a good used copy of Yes Fragile I just purchased, I heard something really strange:

I had on my AKG 702s, fed by my Raptor headphone amp, VAC preamp and a killer analog front end and was listening to one of my favorite tracks, Mood For A Day, which I've heard 100's of times. It sounded beautiful and was completely involving... then, about a minute into the song, I heard a faint click and suddenly it sounded like someone turned on another mic, or inverted the phase, or something, but it was instant and dramatic! It was as if I was A/B-ing two different systems! Different soundstage, presence, one more enveloping, the first more laid back... weird! It lasted until just a second or two before the end of the song.

Of course my first thought, as is probably yours, was that something changed in my system at that moment: tube failure, some circuit that suddenly came on or switched off, etc. But no, I played the track again and it happened again at the same exact spot!

So I pulled out another copy of the album, also from 1972, and listened to the same cut without making any adjustments... it played flawlessly. No phase changes or any anomalies whatsoever.

Have you experienced this? Have any ideas as to what this is? It would be great to hear from any recoding engineers.

Thanks!