I wonder about this myself and would love an informed answer.

My guess, is that because of constant "lawn mowing" older Gazans were killed off at a disproportionate rate, massive poverty tends to increase birth rates, family planning supplies or medications were hard to come by, constant "lawn mowing" convinced people to have more children in the hopes that some of them survive, and pollution or other environmental factors causing premature deaths.

That's just a guess though, and I hope someone with more knowledge chimes in.