Reddit has decided effective July 1 they're going to be charging third party apps for API access. According to the developers of some of the more popular apps, the charges reddit is proposing are astronomical, and will shut all the apps down.
Even beyond regular users using these third party apps for general access, there are third party apps that provide access to the blind that are now in danger of bring shut down. Most mods of the larger subs also use third party apps for mod tools. Reddit has promised better moderating tools forever, and never comes through. More moderating tools are again promised. With a July 1 cutoff for third party free access, no one thinks these tools will actually become available.
To protest against these charges, a lot of subs have decided to shut down through Wednesday, I think.
Here's a more formal write up on "why" :