Possibly finer wire hooks pulling through - compare an 'old' ED treble to the 'new' generation of trebles and you'll see what I mean.
Possibly smaller barbs - (see above).
Possibly that's just the way it goes sometimes
I've had more success with doubles holding over trebles myself, but that could just be as I don't use trebles anywhere near as much as I used to; these days usually singles/doubles.