Forced fork of older, low difficulty blocks
ChainWork is not compared at each specific height. The Chainwork of Chain A in total (at height 1000) would be greater than the chainwork at height 600 if the two were equal at 600.
If the two chains had equal work at height 1000 (the tip), then you'd have a standard block race, as described in the whitepaper. The fork with the most hash rate (Satoshi calls it CPU power), will continue to produce more work and ultimately outpace other forks.