It depends if the brake fluid has been mixed with the previouse, ie dot 5 mixed with lower spec dot 4 or below, it happened to my mates race bike after dot five was added. Or if you mix synthetic with mineral, it is how they react with each other & what they then do to the seals in the calipers, master cylinder.
I am sure Neil or Shazam will come up with a more technical explanation shortly.
Chris.
