I've lifted this article from "Pan Talk" Sept 2016, which is a British e-magazine. I raised an interesting point, so I've copied and pasted the full article here, but it's not something that I've experienced or noticed on new tyres.
On Fitting New Tyres - Are they really that twitchy ?
I always order my tyres in advance, and once they have arrived in stock, I’ll remove both wheels, take them to the dealer and have the tyres fitted and the wheels balanced. During this process, I do various other maintenance checks - cleaning and re-greasing parts of the brake callipers; cleaning up the crud on the swinging arm; oiling/greasing the brake joints on the pipework and the exposed part of the rear suspension; and when I get the wheels back, I check the bearings for roughness and re-grease the drive splines with Moly Paste. All of that sort of stuff. I also check out the clamp bands that secure the exhaust and put copper grease on the threads to help stop the clamp bolt breaking when I come to undo it in a few thousand miles time. With everything secured again, I get rid of the sticky label on each tyre and wait for a fine day to take it out for a ride to scrub in the tyres. I take the opportunity to check that the brakes are working correctly and are not getting hot - always a possibility when they have been removed and replaced. At the start of September, I went through the process as usual and all seems good. I take it easy at first, find an empty bit of dual carriageway and weave back and forth to get different parts of the tyre to rub against the road surface - to try and get rid of the shiny release agent that covers the surface. 25 miles later I decide I can rely on the tyre grip on a roundabout. Strewth. What was that ? The bike twitches as if the rear tyre has caught a patch of ice. It probably wouldn’t look like much from behind, but from the riders seat it feels massive. A little while later, it does it again. I decide to stop, and check things over. The new tyres are no longer shiny except for the rather wide ‘chicken strips’ at the sides. The wheel nuts are secure, front and back. Something smells hot, but it is the copper grease on the exhaust clamps. All is OK. Then I notice it. These tyres came with long rubber ‘spikes’ from the injection moulding. The moulding spikes are quite common, but they usually disappear within the first few miles, and I had ignored them. On this tyre, however, they were about twice as long as normal and although they had gone from the centre, the sides still had most of the spikes intact, so as I was cornering, these thin rubber strands were acting like roller bearings. Once I knew what was happening they were less of an issue - I could lean over, wait for the sideways slip, unclench my buttocks and then lean a little further - but It took 50 miles before they stopped causing the back end to twitch around. I should have given the tyre a haircut before riding
