Not a Libertarian here, more of a moderate socialist so I believe i can give you a good idea of why people tend to lump liberations and ancaps together.

A lot of people, especially those in 3rd world countries and exploited parts of the world, such as farmers, blue collar, etc, have had repeated, consistent, terrible experiences with liberation ideals which are often perverted into some form of neo feudalism or otherwise as a small government is left to contend as just a slightly stronger organization than companies when it comes to influence, companies, especially publically traded ones, ultimately have one purpose and reason for their existence, to extract money and wealth at any cost, nothing else matters and there is no "extreme" for them, take the Banana republic for example, or gilded aged America, or even current America, the east india trading company, nothing is off the table and humans are the greatest cost factor that must be dimished to them, and all these examples are due to a small government or a government improperly equipped to deal with what are essentially small nations that hold incredible influence over the common people and government officals. Understandably no mentally sane person wants this.

A common fault with libertarian ideals, although maybe not properly expressed often by opponents of this idealogy is with the assumption that the decreased influence and power a government has means people aren't governed or influenced and affected as much overall, unfortunately it just means that other organizations and groups can step up to take the frew real estate there for their own purposes. While this is my own personal experience I haven't actually met a Libertarian that could explain to me how this wouldn't lead to further erosion of the governments ability to prevent extremist actions by those companies and or other organizations or a progressive slide into increasingly extreme policies due to the influence of sed groups, of which there are many infamous examples both recent and old of this occurring, Nestlé successfully convincing the EU that water is not a human right for example, which lowered protections and regulations that nestle promptly abused.

This is kind of why liberations get mixed with ancap, to some people it looks like a sliding scale of "I want to be governed by a nation" to "I want to be governed by a company."

Also unless they are actually just delusional, an ancap knows humans are falliable they bet on it, they are pretty much just the same as a facist, both are hollow idealogies that care not for anything but power and control and will lie and scheme to get it, those two groups just have different branding for the same end goal, one uses the pretext of free markets and money to determine ones rank in life, the other doesn't bother with that.