In that sentence burning is not a verb (one of the telltale signs is that there is no object), and so that clause is not a past continuous clause but rather a past simple clause where the word burning is a present participle(a type of adjective) and the verb is was.
as in: He is annoying.
annoying is not a verb, it is an adjective, a predicate nominative not an object.
it might be more obvious in: the burning man struggled for his life.
also from dictionary.com:
aflame; on fire.
very hot; simmering: The water was burning.
very bright; glowing: She wore a burning red bathing suit.