The Constantinople/St. Petersburg hardforks are upgrades only, and don't result in any new or forked coins. There isn't really any reason for Eth to pump, other than complete speculation and probably quite a few people not understanding that there are no new coins to be had.
I think the most important reason for Ethereum's pump was the reduction in block rewards from 3ETH to 2ETH. If you add that people buy what's going up, then it's quite easy to figure out that the price pumps further than it should.
Ethereum is quite complicated and requires consensus on various levels of the protocol, and when even one aspect of a certain large miner doesn't align, a split happens, and it was quite possible in this case.
Bitcoin is super simple--it does one thing, but does it extremely well, while Ethereum is trying to do multiple things well at the same time, but heavily struggles with that. It's not for nothing that they delay upgrades continuously.