iMac 27inch keeps crashing

If rebooting your Mac doesn’t fix things, see if the problem still happens in Safe mode (hold the shift key down at startup). It can take much longer to safe boot (10 min) so be patient.

How to use safe mode on your Mac - Apple Support 

Safe mode forces the OS to clear certain caches and logs and do other miscellaneous housekeeping. It's non-destructive to user data so is safe to do. Very often certain weird software behavior is corrected by simply logging in this way as a troubleshooting method. When you're in safe boot, the machine will not be at its best performance, especially with graphics, but that's expected.

Does the problem persist while in Safe mode?

Exit safe mode by restarting your Mac normally and re-evaluate the issue again.

Also, it could be a software or hardware issue. I recommend that you run an EtreCheck evaluation of your Mac. EtreCheck is a diagnostic app that gathers important info about your hardware and software and reports it back to you. It is very highly regarded and meant to be used with Apple Support Communities to help others help you troubleshoot and resolve your Mac problems.

Please navigate to EtreCheck.com and download the free version of EtreCheck. Once you’ve run the app and created your report please post it with your reply to this message. 

You must upload the full report. To see how, please click >  How to use Add Text when posting… EtreCheck Report.

We can use the EtreCheck report to look for the more obvious things that may be causing your problem and advise how to correct them.

If the problem is still unresolved, please return here and post a reply and we’ll see what else we can come up with for help.

Suggest downloading the Application Etrecheck   https://etrecheck.com  directly from a well Respected ASC Contributor. 

The application is free or paid from added features. 

Run the application with Full Disc Access ( Security & Privacy - Full Disc Access ).

It will take a Snap Shot -  both the hardware and software.

The Report will Not Reveal Any Personal Information. 

Post back the Full Report - copy and paste - using the Additional Text Icon ( 3rd Icon to last )

We can have a look at the report for possible issues and may have possible suggestions to resolve the issues.

Any Third Party Applications that will interfere with the normal operation of the OS, alter, modify, remove or delete or attempt to do so is an invitation for disaster and may require a Reinstallation of the OS.

This includes AntiVirus, Cleaners, Optimizes, UnInstaller etc.