737NGs have cracked 'pickle forks' after finding several in the jets.

[Scuffers]

Originally Posted by Smythe

Watch the winglets bounce around the next time you fly, then watch wings without them....cyclical loading with the extra mass at the end of the moment arm.

Source: leeham
https://leehamnews.com/2019/10/08/bo...pickle-part-2/

so you're suggesting it's fatigue from the wings wagging and the winglets change the resonant frequency?

I can see that, right up to the point that surely the airframe is designed to cope with this? and adding mass to the wing will lower it's resonant frequency?

I don't subscribe to this, I would suggest it' way more likely that a change in manufacturing has caused this.