Why do bees build "burr" comb (comb where the beekeeper did not want them to build it)?

Bees keep ¼” to 3/8” spacing between combs and wherever they want corridors. That is known as bee space. If there is a space less than ¼”, they fill it with propolis, but if the beekeeper leaves spaces between frames, hive walls, other woodenware, etc., greater than 3/8", bees build comb to fill it.