I've noticed this correlation, but I'm not sure if it's just coincidence or if others have noticed this as well?
Necessarily? No. There are plenty of other factors that can slow the pace. Buildup of technical debt is a possibility, though.
Something else to consider is the size of the commits that are being made, both in terms of lines of code and also number of files touched. If the commits have been getting generally larger, that could be a likely sign that technical debt is one of the causes - for example, a "simple" change will require more changes throughout the project in a highly-coupled architecture than in a more loosely-coupled one.
It's also worth looking at the obligations your team is taking on. If you are moving from a system entirely in development to a system in production, many people who have mostly been writing code might now be tied up in fixing deployment issues instead.