I don't have a coherent argument on this one way or another. I don't really hang out in libertarian activist circles or alt-right circles, so I am spitballing here.

I've seen a ton of people I know who were libertarians become marxists; others join the alt-right; others remain libertarian; others move to entirely different, weird positions like monarchism or anarcho-primitivism. Still others just become boring conservatives or progressives. 

Michael Malice's book The New Right is a really good look at the alt-right (a subset of the New Right -- important to note that not all New Right is alt-right) and its association with libertarians. 

Here are a few things that might explain some of this:

Against The Cathedral

I think one thing that makes the alt-right attractive to some folks who are libertarians is that both systems are against what Moldbug calls The Cathedral (Malice talks about this in detail in his book, buy it). As people go along in their libertarianism, some may decide that The Cathedral is run by more than just rent-seeking corporations and lobbyists. That might attract them to the alt-right.

But then again, plenty of once-libertarians see the state as an issue of capitalism, so they become leftists.

For the lulz

It's not unusual for somebody to get involved in a group because they think it is fun. Libertarianism stopped being fun for a lot of people when libertarians kept getting metaphorically punched in the face by The Cathedral on one side and the alt-right on the other.

People might become alt-right because they just think it's more fun to troll 4chan and reddit and the press with 2 edgy 4 me memes. This is particularly true for disaffected young people. What do they have to lose by offending people for a few laughs?

Macho Flashing & Outsider Identity

My friend Geoff Graham turned me on to a great essay a few years ago called the Late, Great Libertarian Macho Flash. This is something you'll see libertarians do a lot: strut out just how libertarian they are by saying stuff that doesn't actually win anybody over during polite dinner conversation like TAXATION IS THEFT.

I think once that gets beaten to death enough, people will go a step further and start saying stuff just to set themselves apart. This is especially true as libertarianism goes mainstream and being libertarian is no longer edgy or cool. Some young libertarians are more concerned about being part of the out-ingroup than they are about winning people over to a set of ideas. So they'll switch to a new out-ingroup so that they can be unique and special again. At one point this might be becoming a Bernie Bro. At another point it might be becoming alt-right. 

Generally, I think it's less *M A N  B E H I N D  T H E  C U R T A I N* than the phrase "libertarian to alt-right pipeline" makes it sound. It has more to do with a selection effect of the kinds of people attracted to both groups. 

I would not be surprised if there were an effort by online white supremacists to recruit former libertarian types -- I don't know. I don't hang out in their circles.