I don't think it was a DDoS, it was just a major increase in traffic.  Imgur started removing political pictures that people uploaded to use in r/the_donald (claiming they would not have removed them if they were not published to the community similar to the old FPH) so they looked for an alternative provider which was sli.mg.  I think sli.mg is up now though, but you can see on their twitter here it was a huge jump in traffic they weren't ready for.  Also the creators of sli.mg did an AMA on the r/the_donald so that brought even more traffic.