I asked around elsewhere, and the answer was that seg fault 11 implied some level of incompatibility between k2pdfopt and MacOS:
https://discussions.apple.com/thread/8217579
I don't get it all the time, but with some big files, I would get it several times in a row, without running anything else except my scrolling software and the finder.
If I could merge pdf files w/o breaking formatting, I could run k2-- sometimes with ocr-- on smaller sections and then merge them together.