I think it’s equally likely that the second version just got a different set of reviewers who randomly liked it more, and the revisions didn’t make a big difference. Having submitted lots of papers to conferences like this I basically think of the reviewer ratings as noise.
For both grants and papers in my experience, there's a "Doomed"/"Not Doomed" threshold you have to get over, but if you clear that threshold things get fairly stochastic.
For *ACL you'd have to justify your wish to change reviewers, though; and you need a good reason for that. I don't know how much reviewers changes for a resubmission are solely due to reviewers' unavailability but it seems unlikely all three of them got removed from the reviewer pool.
