A few questions:
- Does it reboot at roughly the same time of day?
- Are you running automated updates? (You should be.)
- Have you enabled automated updates to reboot the system?
If so your system is not crashing, it is rebooting to install an update that requires a reboot. On Linux/Unix, this is generally a kernel update. On Windows, it could be pretty well anything that can't be unloaded without a reboot.
I have noticed a relatively high rate of kernel updates recently. Good news is bugs are being found and fixed. Bad news, is a reboot is required.
On my system automatic reboots are controlled in the file
/etc/apt/apt.conf.d/50unattended-upgrades. The line that controls it begins
Unattended-Upgrade::Automatic-Reboot and take a
false value. If you don't allow automatic reboots, you should be notified that a reboot is required when you login.
EDIT: There are other packages besides unattended-upgrades. Some of them may trigger a reboot. They should all invoke apt to do the upgrade. Check
/var/log/apt/history.log to see if updates are occurring before the reboot.
If the reboots are occurring at roughly the same time, then there may be something in the crontabs that is causing the reboots.
If the system really is crashing, it may be the BIOS which is rebooting the system. You may want to change the power on state setting.