Not that I know off... Any traffic problems in your city, some problems can really cascade at times? Sometimes buildings upgrading can have effect too. For example if a bunch of your commercial buildings upgrade, they want more customers, but if you don't have population growth/extra zoning to match the increased customer and workforce demand, shops will become unhappy, abandon, decrease general happiness leading to people to leave your city etc leading to less income. Stuff like that.
Sometimes zoning large areas in the past can have effect too, especially residential areas. You get entire waves of people retiring and instead of working and shopping they switch over to visiting parks instead, lowering income. Eventually it can result in a death wave too, causing hearses to clog up traffic, which in turn leads to people and goods not reaching industrial and commercial in time, which causes problems again too. It could really be a bunch of things.