While I'm not sure what "neoreactionary" refers to specifically there are lots of reasons that certain types of liberals see LessWrong as reactionary:

• A somewhat strong libertarian component
• Belief in evolutionary psychology
• Anti-religous (or generally the belief that beliefs can be right or wrong)
• LessWrong's more technical understanding of evidence is incompatible with standpoint theory and similar epistemic frameworks favored by some groups of liberals.
• Those older discussions around PUA where it's presented in a pretty positive light
• Glorification of the enlightenment.