I've had this as well - did a quote a couple of weeks ago and got a reasonable quote of around £500 for a M3 LR. Repeated it yesterday with same insurer/details etc and the quote had doubled!
Fortunately the first quote was still valid and I got my numberplate details through from Tesla, so just rang the insurers up and paid for the policy to lock in the insurance at the lower figure. Doesn't seem to be any rhyme or reason to insurance quotes.
Now got to wait until 25th to pick up the car though....