What could cause swapping when there is no memory spike?
See here for a basic, decent overview: https://www.switchingtomac.com/tutorials/osx/understanding-memory-pressure-in-os-x-mavericks/.

My guess is that something is hitting a bug, maybe doing something like getting into a memory allocation loop or something until that process crashes and gracefully reboots, freeing up that memory. I would check for Chrome updates.

The reason I suspect Chrome is two-fold: one is that I have personally experienced Chrome's resource usage going off the charts, and also because in your screenshot, the 'Force Quit' dialog shows Chrome as being paused - I initially just assumed that was because it was 'the' offender, and that lined up nicely with the theory I already had about Chrome - but in retrospect it could have been not because it was 'the' offender per-se, but the next biggest user of system resources after the kernel itself.