As others have said, pulling them from the target incorrectly is the most likely culprit. Use two hands and pull straight back. That being said, there are other possibilities.
Could be the arrows that are bending are way underpinned. One doesn't always get the spine one orders. Though that's highly unlikely if they're Surewood shafts. They've been right on on any I've ordered.
Also possible that you severely stressed those shafts with glancing shots. Not enough to break them, but enough to weaken them. When that happens, they will continue to bend/soften until they break. Depending on how much they're bending, they may not be safe to shoot.
It could also be due to moisture getting under the finish. Again, some good angular whacks can crack the finish without breaking the arrow. Or, it's possible the finish is thin and/or wore off on those arrows.
Finally, if you've been shooting those shafts for about 5 years, it's possible some have just softened up over the years. I usually break or lose mine before that happens, but back when I could still shoot and didn't break or lose as many arrows as I do now, I did shoot a few until they softened. Of course, some of the other factors I mentioned may have been at work as well.