I have not found the answer to this but it happens almost every day. When using my selection tool (top black now, not the direct selection one) I have always just drawn a box around what I was selecting (because some parts were not grouped/merged) so I can move them all at one time quickly. For some reason now even if I have it all selected, when I go to move it, it only moves one anchor point. Then Illus locks up, I get the spinning wheel of death and lose all my work. Here is a screenshot. See the grey line where it stretched just one point and then locked up. UGH!! Its almost every day. I tried the autosave mode but It makes everything slore and messes with my storage drive so that option doesn't work for me.
Disclaimer: Please note that resetting preferences will remove all custom settings, and Illustrator will launch with the defaults. You may take a backup of these settings from the location mentioned in the article.
Also, check if the files you are working on is saved on an external/network drive or cloud-synced folder. If yes, kindly try moving them to a folder on the internal hard drive like the desktop and share your observations.