I think it's the 9 month sleep regression. My DS does this during every sleep regression, although he's always up for exactly 1.5 hours before he gets drowsy again. If you miss that moment, it's another 1.5 hours before he's able to go to sleep again. He did it at around 8 months.
During sleep regressions, babies' brains are indeed buzzing with learning a whole new way of looking at the world, often with lots of activity to do with physical develompent as well.
There is a great book explaining it called The Wonder Weeks by two scientists who researched all the developmental spurts up to age 2.
They say:
"Your baby may start sleeping less well. Most babies do. She may refuse to go to bed, fall asleep less easily, and wake up sooner. Some are especially hard to get to sleep during the day. Others at night. And some stay up longer both during the day and at night."
With us, it always stops as suddenly as it starts without me doing anything different. I don't let DS cry, even for a short time, as his crying escalates to hysteria pretty quickly.
I let him roam around the cot/bed in the dark while I pretend to sleep if he's happy, and cuddle/rock him if not. After 1.5 hours I rock or feed him and he drops off easily.
CC or even gentle forms of sleep training are unlikely to be successful during a sleep regression period.
More info on the 8/9 month regression There?s useful information about it here, here and here.
HTH