I'd regard alerting as 'something seems to be wrong, but the device can still contact Dashboard', so 'online but maybe degraded'.
Don't know if there's a definitive list of causes, assume things like issues with IP settings, VLAN misconfiguration, DNS problems, switch faults etc.
I've seen devices go into altering state when making changes to network addressing, VLANs etc., sometimes it clears, sometimes they need a reboot to pick up the new parameters from DHCP.