Why would replacing rear tyres increase tramlining?

[Bennet]

I'm in the process of moving from runflats to normal tyres on my wife's Z4. I've just replaced the rears (with Rainsport 3s). The fronts still have a lot of tread so I've left them for the time being.

The car now tramlines on uneven roads far more noticeably.
Why would that happen?

1 - I'm led to believe run flats are meant to be worse for tramlining so this, if anything, should have made an improvement.
2 - It's the rears, so surely there shouldn't have been a difference anyway.

Thanks.

[kambites]

I wonder if you're just noticing it more because the fronts are still following the ruts but the rears no longer are?

[Bennet]

Excellent point.