What Mac OS version is in use? And what other programs are running on your Mac?
Have you tried restarting the Mac in Safe Mode, and see if this still happens?
If you see a black screen on your iPad, when using Astropad while in Safe Mode, this is fine.
But see if the Mac still freezes.
How to restart in Safe Mode:
-Restart your Mac.
-Immediately after you hear the startup sound, press and hold the Shift key.
-Release the Shift key when you see the Apple logo appear on the screen.
To leave safe mode, restart your Mac without pressing any keys during startup.
Please let us know if the problem still happens or not, in Safe Mode.
More on Safe Mode: https://support.apple.com/en-us/HT201262