The issue is the new interiors on the 737s, which have the pivoting overhead lockers needing to be manually pushed up by cabin crew, as opposed to just lids being closed on the older cabins.
This can take two cabin crew to push closed, especially if they are short. If they are doing lots of short flights in a single day, i can see how they could cause injury. You might put your bag in the locker, but the crew close them. Not an issue on the A320s or A330 but these new pivoting bins are not ideal for the crew to close.