Why do some believe the 'assault weapons' started with Clinton?

It should hurt her campaign more than anything. Over 100,000 police officers delivered a message to Congress in 1990 stating that only 2% to 3% of crimes are committed using a so-called "assault weapon. Crime gun traces performed by ATF showed that between 1986 and 1992, assault weapons were traced to 1,578 murders. There were over 140,000 murders between 1986 and 1992, which means only 1% of those murders were traced back to assault weapons. Congress refused to listen to the people though and still passed a ban on "assault weapons." The media pretended that these so-called assualt weapons were actually machine guns and fueled support for the ban. In reality the ban restricted magazine capacity to 10 rounds and banned semi-auto civlilian guns based on comsmetic features that "looked" scary such as bayonets, pistol grips and barrel shroud. It ended up having absolutely no impact on criminals like predicted and eventually expired. If Hillary thinks she has a chance of renewing it she is nuts.

The Manufacture for sell to the public of Automatic weapons of which 'assault weapons' is a subset was banned in 1984. Cannons were and are more powerful. Did you not know USA employed Privateers?

Because the average citizen doesn't have the training or the need for a high powered assault weapons like mini-guns and flamethrowers. Also, if you think it's impossible to defend yourself with a bolt-action hunting rifle, then you obviously don't understand how guns work.

founding fathers didn't mention muskets

Your question doesn't really make sense.