Why does this past tense verb take être instead of avoir like the others?

There are two groups of verbs in French which use être in the passé composé rather than avoir. The first group are the reflexive verbs (think of the morning routine verbs se laver, se lever, se doucher) and the other group are Mrs Van Der Tramp. However, you must be aware that when using être as the stem, the past participle must agree with the subject. For examples if it's a feminine subject then you must add an E to the past participle and if it's plural you must add an S.