I get the same thing problem on RB750, RB450, RB1100.
It also happens if the state of a ppp interface changes, 95% of the time it reboot and hangs on, you have to remove the power to get it working again
We too are seeing this : Just rolled out 250 3G units for coaches, and have the system hang randomly when the PPP interface is reset. Crashes the console too when you type /int ppp-client print
Tickets and Support files, along with full export and all scripts sent to Mikrotik. Upgrading to 6.11 Did not fix for us, in fact, issue is worse as the usb interfaces now randomly disapear and strange ppp-client entries appearing.