For anyone who is curious for the actual reason why Donald Trump's AMA decayed off /r/all so quickly, let me explain how karma decay works to the best of my ability. However, the /r/all algorithm and anti-cheat algorithms aren't fully available in reddit's source code, so some things are a bit speculative and based on what they've explained to us.

What factors play into karma decay:

1. Karma per minute / age of post. Getting lots of upvotes early will make it rise more quickly, as the post gets older, the more quickly the karma will decay
    
   [How it affected Trump's AMA]
   When posted, it got some traction right away, but didn't actually spike until he started answering questions about 45 minutes later. However, especially on /r/all, new posts take much greater priority especially after they modified it back when everyone was complaining that posts stayed on /r/all for too long and important news wasn't getting to /r/all quick enough. However, it can be argued that this system kinda hurts AMA's since they have a second activity spike significantly later in the posts life.

2. Score, upvote ratio, confidence rating. In addition to the score and age of post, reddit also gives a post a confidence rating which does a bunch of math to account for upvote ratio in the equation in a manner that makes the most sense.
    
   [How it affected Trump's AMA]
   Although the post had a lot of upvotes, it was also one of the most controversial threads ever as well. /u/spez mentioned this a bit in his post as well. This was also a significant factor in it's decay.

3. Soft-capping extremely-popular posts. Another thing reddit's sorting algorithm does is 'normalize' extremely popular posts so that they decay properly and don't persist way longer than they should. They've recently made several adjustments to this and agree that it's a flawed system, but they rely on it too much as of now to make significant changes to it.
    
   [How it affected Trump's AMA]
   Obviously, the upvote ratio was significantly higher when just seen by subscribers and people viewing /r/the_donald, but was greatly downvoted once it got to /r/all, however, since it hit the soft-cap fairly early on, the votes that came later had a bit more weight than they would have pre-soft capping and since those vote came more from /r/all, they were more negative overall. You'd be right to be upset about this factor as it's clearly flawed, but it's been in place for years and not anything new. Definitely not some conspiracy against /r/the_donald or Donald Trump.

4. Same-subreddit post decay on /r/all. A recent change to the /r/all algorithm gave less weight for each sequential posts from a subreddit on /r/all to provide more diversity. So if a subreddit has several heavily upvoted posts, each one after the "hottest" post gets a decayed /r/all ranking.
    
   [How it affected Trump's AMA]
   Almost immediately after Trumps AMA started to decline from the factors listed above, /r/the_donald started mass upvoting dozens of threads being outraged that it decayed so quickly. Those threads started becoming more "hot" than Trump's AMA itself, thus hastening Trumps AMA even more quickly than before! So effectively, /r/the_donald themselves played a fairly significant role in decaying it off /r/all. (And considering they know about that change, they should have known better. Sad.)

Note: A lot of them cite the number of comments, gild, comment upvotes and other factors as reasons why it should have still been on /r/all, but those don't play a factor at all. Participation is not a factor in calculating ranking.

All in all, however, you can make some good arguments as to why Reddit's ranking algorithm is flawed, but this was not some big conspiracy orchestrated by the admins as /r/the_donald would have you believe.