To be clear, Mike Hearn's unilateral fork doesn't switch over to the new rules when 75% adoption is achieved, or even when 75% of mining is done by supporters - it switches when 75% of the last 1000 blocks indicate support, which can quite easily happen with less than 75% support just by random chance. This is an astonishingly aggressive threshold, and if he succeeded it's almost certain that Bitcoin would fork into two different, conflicting versions of the transaction history for a substantial length of time.