I read about the disabling SNMP, tried it too, didn't help for me.
This is by the way what the CPU looks like after the reboot. Busy, but normal:
Yes this is exactly what happened to us. In our case it seemed to start when a winbox session terminated abnormally that had a large table open (I think it was the ip routes table). The user session showed as permanently active on the router (under system->users->active users) even though it had disconnected. My guess is that the management CPU was high because it was trying to redraw the table every single time there was a change, even though the user was no longer connected.