SO is I/O bound most of the time. If you've set up your system to handle high workloads of I/O bound traffic, then hitting CPU bounds throws a real wrench in your cogs.
To put this another way, SO is one of the most traffic'd sites on the internet. So a page that's loaded 10k+ times a second is going to push that number much, much, higher. If the CPU can't clear 10k+ req in under the regular time it takes, everything starts to back up.
Alexa puts it at rank 50 globally. Honestly this seems suspiciously high and I wonder if their sampling method is biased. Regardless, it's a lot of traffic.
To put this another way, SO is one of the most traffic'd sites on the internet. So a page that's loaded 10k+ times a second is going to push that number much, much, higher. If the CPU can't clear 10k+ req in under the regular time it takes, everything starts to back up.