Commerce is relatively easy content but requires relatively more keeping up, motivation to study at home, etc.
So whilst I'd say any reasonably intelligent person who managed to get in via HECS should expect a D, some don't achieve this as their mind is elsewhere.
HD's are hard not because the content is hard but because you're likely to lose marks from anal marking, random ambiguous questions and group assignments.
That said there's also a lot of drop-kicks studying commerce. But that's a separate group altogether.