Interesting. I tried this out on my MBP and I can reproduce the issue sometimes with Safari. It seems to happen when scrolling up and you hit the top of the page or scrolling down and you hit the bottom of the page. After that, scrolling stops working until, like you said, you lift your fingers off the track pad. It doesn't always happen, but it happened most of the time for me. I hadn't noticed it on my own since I'm normally lifting my fingers as I continue to scroll down a web page.