Hi Ross, this is perfectly normal if too much is added too fast.
Also by the way, I run my kalk on a drip system for 2-3 days out of every month and a day or two after the drip cycle has finished my PH returns back down to 8.2
Just be careful, as a fast PH swing can be really stressful or fatal on not just fish, but inverts and corals too
That's why it's best to add a disolved mixture of kalk and top off water by drip line or by adding small amounts of disolved kalk into the sump, rather than directly into the tank.
cheers, Rich