I think a lot of Reddit's earlier userbase was more lefty types and that also overlaps with the group most annoyed by the API changes since they'd gotten used to what Reddit used to be. So more of the left wing users left, resulting in the attitude shift you're seeing. Even the right wing users on early reddit were more right-libertarian than right-conservative or alt-right initially - Ron Paul was one of the first political figures to have a big following there.
That said, there is also a shift to the right in real life, as seen by the rise of groups like AfD, Brothers of Italy or Sweden Democrats.
r/europe also has a bit of a weird history with this too. It started relatively left wing in the early days of reddit, drifting to the right until the mods started clamping down on the more overtly racist stuff. At this point, a lot of the posters involved moved to r/european, which sort of reset r/europe back leftward a bit again. But then r/european was banned around the same time as r/the_donald for much the same reasons, and those users drifted back into r/europe which shifted it to the right a second time.