The change is a minor one, but the rationale has been documented in a BIP for posterity. Prior soft forks were activated via miner signaling in block version numbers, but now that the chain has long since passed the blocks at which those consensus rules have triggered, they can be replaced by caching the block heights at which those consensus rules became enforced. The purpose of this change is to simplify and optimize the trigger mechanism. Eric Voskuil, however, has expressed his disagreement with this proposal, calling it a horrible precedent, an extremely poor process, and not even a material performance optimization.