imgur rewrites them so they can serve you adverts, and modal dialogs telling you to install their application.
imgur started out well, as a fast and simple image-host, but like all the others it tries to recoup money by serving adverts and getting up to increasingly shady things.
Soon it will be time for the cycle to repeat and a new host launch itself; no doubt with a cute tagline saying they'll even host nsfw content.
I didn't realise they had a traffic-based condition that did this, too. Wow.
They've been pretty hostile with this sort of thing for a while. Look at this behaviour:
# Mobile UA
$ curl -v -H "Accept: text/html,application/xhtml+xml,applciation/xml;q=0.9,*/*;q=0.8" -H "User-Agent: Mozilla/5.0 (Android 10; Mobile; rv:68.0) Gecko/68.0 Firefox/68.0" https://i.imgur.com/6WAZZpN.jpg
< HTTP/2 302
< retry-after: 0
< location: https://m.imgur.com/6WAZZpN?r
# Desktop UA
$ curl -v -H "Accept: text/html,application/xhtml+xml,applciation/xml;q=0.9,*/*;q=0.8" -H "User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:71.0) Gecko/20100101 Firefox/71.0" https://i.imgur.com/6WAZZpN.jpg | file -
< HTTP/2 200
< last-modified: Wed, 18 Sep 2019 20:45:26 GMT
< etag: "21d323ddb1c1926ab0d7a5bb9786c996"
< content-type: image/jpeg
...
/dev/stdin: JPEG image data, baseline, precision 8, 2250x1500, components 3
They use UA sniffing to disguise this behaviour from technical users on desktop. They also detect if you're using curl and spoofing a mobile UA (which sends Accept: * / *), in which case this dark UX pattern is disabled.
Not only is this a hindrance to users but it actively blocks archiving.