This is just more prodding the weak points of the infrastructure by a state-sponsored group. Someone is testing the IP stack, from top to bottom, and looking for weak spots.
(Which is, admittedly, a bit self-defeating, because once these weak spots are exposed, sysadmins generally try to harden them. But, those who do NOT harden their defenses after being "poked" like this, are all the more weaker the next time. So someone may simply be testing to see which parts of the internet infrastructure are being actively maintained, and which aren't. This is also extremely valuable information if your goal is to really damage an organization/group and take them completely offline for more than a couple of hours.)
Of course, if you wanna go all conspiracy theory, I have another take on KB's comment: USCYBERCOM is basically hardening the US internet infrastructure by sticking its fingers in these holes, and causing companies to strengthen their security. You can pass any law and regulation you want, and make recommendations for how companies need to secure their shit... but until you actually take their system down, the bean counters at Generic US Company, Inc are never going to approve those budget expenditures.
But when your web site is down? Oh yeah... they'll authorize the spending to fix the problems. So maybe USCYBERCOM is delicately forcing US companies to harden their security...?