A section of the General Laws, I believe it is Chapter 93A, gives the AG the authority to develop regulations that protect the consumer from unfair or deceptive business practices. It is very common in government (both state and federal) to pursue a policy goal through regulations rather than legislation simply because it can be easier. Environmental agencies do this sort of thing all the time. They will write and enforce regulations that go beyond the letter of the law. The legislature and or governor/president don't say anything about it because it allows these policy goals to implemented without them having to put their necks on the line.
So at some point a group of people realized that since guns are "dangerous" they could be regulated by the AG under the "consumer protection" law. After all, us consumers don't want to get lured into buying those kinds of guns that go ahead and shoot themselves. That would be dangerous for the children. This allowed the "too dangerous" guns to be banned via regulatory fiat instead of having to go through the normal, democratic legislative process that nanny staters find oh so burdensome.
As to the specifics of the regs such as the 10 lb trigger pull and other nonsense, it was probably whatever was easiest at the time to get past the sheeple at large without going far enough to make people question their intent.