What’s the reason for this clawing?

Looks like a heat issue to me as the lower leaves look fine, only the top leaves affected. If the claw was at the tip of the leaf blade, then its excess nitrogen. I would raise lights, increase air intake (if possible) and air flow through your grow space. Make sure your fans are oscillating and not blowing direct. I run a 1.5 x 1.5 and have 3 oscillating clip fans running alongside my in-line intake & extraction fans, so 5 fans total running 24 hours. If growing in coco and not maintaining constant saturation of media around 90%, I would also flush for good measure and start with a clean slate. Happy growing!