If you take a look at the 3rd printing of your owner's manual, pages 151-154 go over the various climate control settings and how to best utilize them. If you are concerned that your C-Max is not operating in the way it is described in the manual, make an appointment to have your dealer take a look and let me know. I'll see how to best assist.
I just read over the relevant pages in the 3rd printing of the manual. They don't answer the question.
I have had the same thing occur in my C-Max. I drive with the AC off. I park and turn the car off for a while. I get back in the car and restart, and the AC comes on with the fan at full blast. Or, I have the AC off and I turn the climate system off before I turn the car off. I restart the car and turn the climate system back on and the AC comes on with the fan at full blast. I don't remember all the combinations.
I have found that turning the climate system back on by pressing one of the fan buttons seems to avoid having the AC come on and keeps the fan speed where it last was. I don't use the AUTO setting for my climate system, but I wonder under some conditions it goes to AUTO by itself, senses a high cabin temp from the care being parked in the sun and goes into AC with high fan speed.
Mostly I consider the AC part of this an annoying quirk so long as I catch it before I get frozen and before it depresses my mpg. The high fan speed is more annoying to me because it is an unexpected loud noise.