Typically what you see are "amplification attacks". That's where Alice wants to DOS Bob, so she spoofs a request to Charlie that appears to come from Bob. This results in a message from Charlie to Bob. The message from A->C is crafted such that it results in a much larger return message from C->B (hence "amplification"). That lets you create an attack that produces a multiple of the bandwidth that you actually control. Then you have a bunch of machine spam the message.

In that case, you see many messages from the same source address (meaning the target under attack, i.e. Bob), but the data requested may vary. In this case, the source addresses were uncorrelated, but they all wanted the exact same address, so basically the opposite.

I can't say that I know what it is, but when you see massive spikes like that it's usually a botnet of some kind (whether it's infected machines, injected connections, or whatever method). Perhaps a bunch of bots resolving their next C&C master?