What was the major impetus for reform between the 1820s and 1860s?
The major impetus for the reforms of this era was the change that was happening in the US economy and, therefore, in its society.
During this time, the US economy was becoming more of a market economy. This is one in which people create products to be sold and then use the money to buy things they need. This is as opposed to the older system of subsistence in which people generally produced most of what they needed for themselves and then did minor trading with others near them.
As America's economy moved to a market economy society moved from being then neighborly thing where people knew and took care of one another. It moved towards the more impersonal society we have now. This change led many people to try to "fix" things. They felt that their world was changing and not for the better. Because of this, they were motivated to push for reforms.