I was using IMDB pretty regular and its blocking seemed to coincide with its availability of movies and videos; it went from only info about movies, to also a movie site where you could watch entire feature films for free. If you look at how other video sites have all been blocked in recent years in China (yutube, vimeo, break, etc) and also the political developments of the use of social media with alphabet soup influence, I think this is the underlying reason why the firewall blocked it.
Also, look at how the internet archive got blocked for a while and when it came back its url structure (from my casual use, I may have missed existing trends) included www.archive.org/blah/DETAIL/url
where you can not access these "detail" archive links that have videos, but you can access most other places on the site.
I think it's all about China wanting to protect itself from social media and related content that it cannot control. Another example would be a while back around the beginning of the Arab Spring and a certain message showed up on many Chinese discussion boards with a certain flower and telling them to go to McDonald's and have a meeting, Huntsman showed up by coincidence, blah, blah.
It's been so long since I used IMDB I can't remember where to go to watch films.
I think it's that kind of stuff that is making them crack down on any video sites, but it's great to have imdb back 'cause I use it all the time.