Beenz

"About 1 month ago I changed my rear Z6 for a BT021. I still have a Z6 on the front that has done about 5000 miles, so it is now worn flat on the sides, although there is still about 2mm of tread pattern."

Do you mean its worn the middle of the tyre?

Once the profile on any tyre has gone, leaning off the perpendicular the wheel gets more and more nervous untill past the middle portion. 5K from a front can shag the profile, certainly considering the less than perfect road surfaces over the winter. As the bike leans the contact patch goes through a reduced level before it rolls onto the good part of the profile.

It's testemant to your riding that you managed to catch a front end slide, quite often that results in a swift journey to the scene of the crash.

I'd change the front boot to match the rear.