Imgur is reddit's go to site for image hosting, and when an image get's enough hits from an outside source the image goes to Imgur's front page gallery.
A subreddit with 150,000 subscribers easily has the numbers to consistantly generate enough views for their content, which would previously be private to those with the URL (AKA, those viewing it from FPH), to get to the front page. The Imgur staff didn't like this, they started removing images that appeared from FPH, I don't think FPH would give a shit but the issue was that the images weren't just removed from the imgur's front page but from Imgur entirely, meaning many FPH submissions linked to nothing, which irritated FPH.
There were two types of responses, use a different image hosting site (I think one of them was named slimgur.com) or to allow the Streisand effect to kick in and upload as much FPH as one could to Imgur, often aimed directly at the staff. This was a cue for Reddit admins calling harassment and banning the sub.
Attempting to remove any personal bias from the situation is difficult, but you can see both sides. Large parts of the FPH community were breaking the rules by deliberately uploading offensive images aimed at the image hoster's staff. However it was definitely a retaliatory move, a typical lash out from a subreddit so big and by far and away not the biggest relation that reddit has hosted, I mean there have been a few reddit scandals over the years and there has been a lot of targeted harassment between subreddit and between other sites, FPH vs Imgur is very small in the grand scheme of things. I mean shitredditsays seems to make harassment against other redditor's their main activity whereas FPH explicitly banned linking other parts of reddit for any purpose, so it seems to me that the Reddit admins were simply using this as Casus Belli.
Although, it is a classic pump and dump scenario and discussing this outside reddit when a lot of people reading this don't use it, seems a little off, considering most people reading this won't understand the internal politics of the site, not that I ever read FPH, I stick to UK, science, history porn and programming subs.