What would cause draft beer lines to freeze?

Question

Last night when I went out to eat with my parents, we found out that all the restaurant's draft beer lines had froze. The waitress said it happens at least once a year when it gets cold. What would cause something like that? The waitress mentioned something about it being the way the building was designed. Are the lines not insulated enough?

Answer 1

temperature

Answer 2

they got real cold!