It's something I've heard here and there from history teachers growing up. I know it has to do with him not really doing anything to quell the tensions leading to the civil war, which I think started immediately when Lincoln took office iirc.
I'd like to get a better picture of his ideology and mainly his intent: was he just a doofus who didn't realize how imminent a civil war was? Is there some political or financial gain he expected to result from his inaction?
Disclaimer: I will be comparing the responses here to a more recent presidency that is not at least 20 years old. I just want to be up front that this question is not asked in the purest of faiths.
While hopefully someone comes along to address your questions specifically, in the meantime, you may want to read this previous post by /u/Red_Galiray, who gives a good summary of Buchanan's actions as president, and why they so often get him labeled as the worst president in U.S. history.
On one specific criticism of him, I have addressed in more depth his involvement in the Dred Scott case.