Looks like all services are working right now. Looking at the logs, I can see that the kernel killed a couple of its threads due to them hanging. Both threads were in codepaths that, guessing from the stacktrace, were doing some operations on the SD card. This, together with the diagnostics @sradevski referenced above, indicates that the SD card is getting overloaded.
This could very well be the root cause of the supervisor restarting: slow SD card IO while downloading an image (which is IO-intensive) triggering the kernel’s hung task checker. The supervisor would then be killed and restarted, bypassing the watchdog. However, the supervisor hasn’t restarted since the last reboot and all services are already updated, so there are no logs to confirm this hypothesis.
It would be best to replace the SD card (from our experience, the Sandisk Extreme Pro works very well). If that is not possible, you could try increasing the kernel’s hung task timeout: see http://beautifulwork.org/hung_task_timeout_secs-hung-task-timeout/ for an example of how to do that.
Please update us if this solves the issue for you.