Not only were some mining pools SPV mining, but they were advertising compliance with BIP66 at the same time; if they had not been advertising compliance, then it is unlikely we'd have switched over to enforcing BIP66.

So, don't lie about your SPV status in your coinbase is one of the lessons here for a pool operator.

When I ran a (small but noticeable) pool, the bitcoin core team would occasionally reach out with warnings when an impending network change was coming that we didn't look compliant with; often we'd get two or three e-mails independently with plenty of warning.

So, I would say this is chinese mining pools not playing by the rules, either because they didn't care to listen, or didn't care to have engineers do anything about it, and now they are eating it a bit, (and of course we all have some minor knock-on effects for a little while).