As someone who follows /r/ShitRedditSays and /r/SubredditDrama on and off I find it completely unsurprising. For
years reddit has been fertile recruiting ground for racists, sexists, anti-semites, homophobes, transmisogynists, and ableists. This is largely the fault of Reddit's administrators who had a strict laissez-faire approach to content on their site and refused to remove content that was overtly any or all of the above. Hell, getting them to remove /r/jailbait (which, even if the public posts were strictly legal (a few posts were proved not), provided a convenient meeting place for actual pedophiles who would trade illegal content over private messages) required the attention of the mainstream media! In 2015 they created a Terms of Service that bans "involuntary pornography" and content which "encourages or incites violence" or which "threatens, harasses, or bullies" but before then they did not.
The admin's attitude towards speech on their site fostered a strong contingent of posters who posted edgy "ironic" non-politically correct content in what was a relatively safe space to do so. Having a place where this type of speech was normalized attracted people who actually believed those non-PC stances and they posted "ironically" as well until the lines between irony and un-irony completely broke down. A very good example of this is /r/fatpeoplehate. Before it was banned it was one of the fastest-growing subs and their horrible bullying "memes" permeated the site ("found the fattie!"). After it was banned there was a brief shitstorm in which FPH subscribers flooded the site with FPH content but after a few weeks they largely disappeared from the site. Fortunately FPH was nipped in the bud; other hate/bullying groups are established enough on the site that even the removal of their dedicated subs (like the Chimpire network) did/will not drive them off.
You may have also been under the impression that Reddit in general (as much as that can be a thing) was somehow "progressive." This has some truth to it but the events of the past few years have mostly exposed this for what it mostly really was: "brogressiveism." Basically, on any issue that would impact young white middle-class men Reddit will generally hold progressive views. This includes gay marriage, drug legalization, college tuition/student debt, etc. But when it comes to issues that do not affect that demographic Reddit becomes far more conservative. The blistering responses to Black Lives Matter (about racial inequality) and Tropes vs. Women/GamerGate (about feminism and representation in video games) demonstrate this nicely. (That's not to say there are not people who are genuine progressives on Reddit; there most definitely are but they are far outnumbered by "brogressives" and outright conservatives on the most popular subs.)
/r/The_Donald combines both of these traits. It may have started ironically as a way for people to make jokes about a joke candidate, but just as his campaign became serious so too did the "jokes" on /r/The_Donald. Additionally, Donald Trump has enough brogressive views that he can appeal to that crowd without the inconvenience of challenging them to consider people who have different life experiences than their own like true progressive candidates would. Basically, /r/The_Donald is a perfect confluence of issues that have been underlying Reddit for years.