Well, if you planted from a pot as opposed to bare-root, its possible the grower got sloppy and dropped a red one in.

Otherwise, I don't know a lot about dahlia genetics but do know that they come in a variety of colors, including red. It could be a mutation that effects only part of a plant which are known as "sports", particularly if they take the plant in a desireable direction. It could be that the plant was bred from a red ancestor and has sported back to something truer to its ancestry, in which case it is less desireable, more common and more usually referred to as a "reversion".

Interesting! Thanks for the information.