The explanation comes in three parts:

Static electricity

Whenever two dissimilar materials are placed in contact, a few electrons will move from one to the other. When the materials are separated again, the charges get left behind. Although the amount of charge and energy that are involved is small, the voltages can be surprisingly high -- thousands of volts!

Getting rid of static electricity

Solid surfaces attract water, and in high humidity actually have a thin film of water covering their surfaces. Even though this is not visible, it serves as a conducting path which carries away any static charges that are created. When the humidity inside is low, the conducting film does not occur, and we start getting static shocks.

Why the air inside has low humidity in the winter

The amount of water the air can hold depends strongly on the temperature. In the winter, the air outside has very little water in it. The air inside your house is the same air, warmed up -- but changing the temperature does not change the amount of water in the air. So the air inside your house is likely to be very dry in the winter. This is undesirable, not just because of the static problem. It isn't good for the African Violets, and it probably isn't good for you. It means that you are being cooled by evaporation all the time.

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