Maddam: Most blitterblats have a fairly high CofG .. engines, transmissions (main and engines), shafts, swashplates, hubs, heads, and rotors etc. all typically mounted above the cabin .. but this makes them 'vulnerable' when being towed more than during taxi. I say 'vulnerable' but the risk is minimal and I have never actually heard of an aircraft being rolled while being towed .. although I have seen them collapse while towed into or across something they weren't meant to encounter!
When taxiing (ie. under power) that 'upper weight' (ie. high CoG - often enhanced by low fuel and nil pax) is mitigated by the gyroscopic effect of the main rotor.
However, if you were to taxi as fast as you could and then boot in full rudder .. perhaps you could 'trip it up' but in fact if you did that .. you would most likely achieve a 'tail swing' which might (given how the 139 'hangs') result in a boom strike which could result in a roll over.
Well that's my thruppence worth but you should be advised .. I'm no expert on such matters!
ps: Speaking of gyroscopic effect .. it is possible (so I understand) to misjudge a run-on landing (which if you've had a brandy for breakfast could be interpreted as a fast form of taxi) whereby if you 'strike' one of the craft's wheels extremely hard it may cause the fuselage to rotate and if one of the wheels were to 'catch' any sideways movement while not properly planted on the ground .. then you will get your roll over.