This kind of subverts union leadership though. How can the union leadership negotiate in good faith when there's no evidence that they are in a position to control or act on behalf of members?

Then again, I think it's technically illegal for members of the airline industry to actively strike. So maybe this is how it's done for plausible deniability sake.

The rumors are that it's due to the employee covid vaccine mandate.

I think that's the reason for the wildcat strike - the union won't abide by it so a group of pilots who don't want the vaccine are not working to make their point to management. It's all hearsay though.