Recording is an art not a science. Someone will probably say this shifting is due to your particular room acoustics, and so you need to hire a consultant and buy a lot of panels and stuff. But if it was the room you would be saying this happens a lot. Instead of "occasionally". If its the room or your setup then it will happen not with any one particular instrument but with anything within a similar frequency range. This I am pretty sure is not the case or you would have said so.
Not at all uncommon. Especially not with multi-track studio recordings. Can't think of anything like this happening with small ensemble or quartet or solo type classical, but does happen all the time with pop, rock, etc. So the simplest explanation is it sounds the way it sounds because that's how they want it to sound.
