It's not rocket science. The overhead lockers are built to carry a certain load, and not collapse under the aircrafts rated g-limits, probably +4/-2g. Under 4g a 5kg bag weighs 20kg, a 7kg bag weigjs 28kg, and a 13kg bag weighs? Yes, you got it, a whopping 52kg!
Now imagine two rows of 3 passengers each, all with overweight bags in the locker overhead, let's say 10 kg each. That's 60kg, under 4g becomes 240kg. The locker is only rated to 6*7kg*4g = 168kg. What happens to the excess 72kg? It breaks the locker, and dumps the contents (240kg!) on someone below.
Not all aircraft lockers are equally strong, and I have used the 7kg limit. An A380 may be able to take 23kg, an ATR or a Caravan can not, and possibly budget airlines with denser seating patterns use weaker lockers.
There is of course a designed-in safety margin above and beyond the rared limits, but - In a real emergency, the aircraft may well sustain g-loading well over the rated limit. Something will definitely break!
I will not bore you to death with the small matters of loading, weight and balance, suffice it to say it is well within the capabilities of overweight, in all senses, passengers and their baggage, to render an aircraft unsafe if rules are ignored.