Usually celebrities trend on Twitter because they died, retired, or are One Direction. None of that describes Back to the Future star Michael J. Fox. So why are so many people tweeting about him?

The Parkinson's Disease-suffering actor was the tenth-most discussed topic on Twitter in the U.S. this morning, behind other, more timely topics like Rosh Hashana, Derek Jeter, and #SkinnygirlSnacks. According to the site trends24, over the last 24 hours or so he's risen as high as the number-three spot, and he's been trending on and off for three days.

A quick search for Fox's name reveals the explanation: for some reason, an army of bots—that is, spammy, ostensibly automated Twitter accounts—is incessantly tweeting bad Parkinson's jokes about him.

Michael J Fox is good at acting but GREAT at developing Polaroid photos — Ayanna Carter (@hoakace1984) September 24, 2014

I'll make the cake if Michael J Fox does the sprinkles. — Lachelle Carter (@giahiov25) September 24, 2014

In retrospect, it was a poor decision on my part to ask Michael J Fox to hold my balls while I hung from a trapeze bar. — Uriel Perry (@litaragn) September 24, 2014

Some of these youtube videos are shakier than a hand job from Michael J Fox. — Yvone White (@yvohwhitei90) September 24, 2014

Every few seconds, a new one pops up. Jokes get recycled verbatim. Occasionally, they're not jokes at all, but sentimental tributes or inspirational quotes. In nearly every case, the bots sprang to life earlier this week, tweeted a few inane observations—usually cribbed, word for word, from someone else's feed—then get to the Fox joke. Many of their profiles are formatted the same way: handles like gunnarl30, poeweqi24, and fuapode84; photos that appear taken from other people's public social media accounts; locations written out like "Napoleon town, IN, USA," or "Winona city, MS, USA," or "Swinomish village, WA, USA." Their Twitter bios are snippets lifted from authors like Robert Benchley and George Santayana.

Whatever is happening, it started happening this week. A search for Fox's name on the social analytics site Topsy reveals an explosion in popularity beginning Sunday.

He's also not the only celebrity to suddenly become one of Twitter's most talked-about:

Who would go to the trouble of creating what must be an enormous fleet of Twitter bots just to tweet terrible jokes about a not-even-particularly-famous celebrity? Trolls? Assholes? Who's got a grudge against Michael J. Fox?

Probably no one. Twitter has long been home to large armies of fake accounts, each tweeting the same single line; it's not hard, searching around Twitter, to stumble on a small group of these bots, each sending out its own copy of, for example, Gawker writer Jordan Sargent's Twitter bio. The various bot swarms sending out Fox punchlines are different only in their size, and in the common subject of their copied tweets.

So who is behind it? The most likely theory we've heard is that the accounts themselves were created by the many shady companies that sell Twitter followers—companies whose business practices appear to involve creating huge hordes of accounts that seem just real enough to avoid Twitter's spam detection services. The best way to appear real, of course, is to scrape the real words of real people—like, say, a bad but decently popular Michael J. Fox joke.

[h/t Neetzan Zimmerman/Image via AP]