This is part of the standard boarding PA. And to be fair, it's your, (and that's a general "your"), often heavy hand luggage in there. Would it really hurt you to close the locker when you've finished putting your stuff in? I've lost count of the number of times I have struggled to shut one, whilst the big beefy bloke that has filled it, looks on without even attempting to help.
The worst types of lockers are the 777 ones as when you are closing them you are pushing the full weight of the contents of the locker up with only megre pneumatics in them. If a crew member had to close up to 20 of these per sector I can assure you that said crewmembers would have serious lower back problems and RSD in their wrists in later years all because they are clsoing lockers with someone elses stuff in it. We ask you to close the lockers for much the same reason that we do not place your baggage into the lockers.
I had to fill out an incident form over the summer because shortly after take off on a 777 my left wrist (and Im right handed before anyone starts suggesting naughty things) swelled up and was really painful as a result of closing lockers before take off despite following set down health and safety practices for doing such.
Its not really fair for passengers to assume crew members should lift luggage that is not their own...at the end of the day, you pack it you stack it!