I can tell you I solved this issue, but I can't tell you why it worked. I'll tell you anyway in case it helps; for me this was on a Late 2015 27" iMac with 3 TB fusion drive. I experienced this issue for days or weeks at a time, often with less than 30 minutes between crashes. I ran hardware tests on my drive and RAM, I minimized my application usage, nothing seemed to reliably help. Eventually I noticed that after restarts my Finder windows in the status bar at the bottom were showing very low available HD space (e.g. 50 GB left on a 3 TB internal drive). I thought maybe swap space was an issue and would delete or move files to a Western Digital FireWire external HD when the computer rebooted. Disk space appears better now (80 GB available again), so yay! Then the iMac crashes again. On rebooting, it now appears that there is just 50 GB or less space left. Huh. Delete, repeat, rinse. Hey, I am up to 100 GB free. Crash. Reboot. Huh, I only have 50 GB free again. I checked for crash dumps or other kinds of new big files and found nothing. And then it would crash again and I would find myself back around 50 GB disk space available. Like somehow the OS had lost track of what was free disk space and started writing swap into bad places? I have no idea. But through all this weirdness no actual data seemed to be lost. Eventually I went through an arduous effort to find and move all the big files and directories that didn't need fusion drive-level performance until I cleared about 300+ GB of free space. Once I did that, the available disk space reports seemed to stabilize and I haven't had any crashes since then. (I also disconnected the WD FireWire 800 external HD and replaced it with a USB 3 Fantom Drive, but not sure if that's related.) So I can't tell you why my chronic issue here stopped, but I can tell you what I did to stop it. Good luck with your issues!