RF Attenuator Failure
You have exceeded the pulse power handling of the attenuators.

Some resistors, like bulk water, or wirewound, dissipate the heat in a very large volume of material, so can have very high pulse power ratings. 

Some, like thin film, have a very small volume of resistive material deposited on a more-or-less thermal conductor. Unfortunately, the thin film requires heat to be conducted away fast enough to keep its temperature reasonable. 

The difference between 666W and 600mW may be key to your problem. I often exceed average power, but by a factor of a few, or maybe 10. More than a ratio of 10, you really need to look into the pulse energy rating of your resistors. You are trying for a factor of 1000!

Now some attenuators, and I know this from when I blew them up as a junior engineer, have a profile that concentrates the heat in just one tiny part of the film, and they drop like flies with the merest pulse overload.

You need to multiply 666W with 300uS, and find an attenuator rated for that number of Joules, as well as the average power.