Chickens 'walking like penguins'

I feed nextdoor's chickens when they are on holidays. Yesterday they were telling me about a mystery illness that had afflicted one of the chicks. The only way they could describe it was that the chick was walking like a penguin.

Anyone had similar experiences? An ideas as to cause?

This happened in the cold weather, and no, this isn't a wind-up!

Apparently she recovered after lots of tlc and being brought indoors in the warm for a few days.