Hi Rust - there are two versions of the verb monter: the first one works with a preposition (sur, en, dans etc) between it and some object (the thing one is getting in or on); the second version works directly with the object (with no preposition between).
If there's a preposition, then you will use être as the auxiliary, but if there's no preposition you use avoir as the auxiliary.
This is something that is commonly misunderstood and wrongly taught by non-native French teachers who assume that monter always takes être as the auxiliary. There's even a famous mnemonic diagram called the 'maison d'être' which shows monter and descendre with stairs, implying that être is used in passé composé sentences like '... monter les escaliers', but we use avoir in this case.
Hope that helps!
If you're interested in learning a little more about the grammar behind this, it's worth getting familiar with the terms 'transitive' and 'intransitive' verbs:
Don't worry though if grammar jargon is a turn-off; all you need to watch out for is whether a preposition is bewteen the verb and the object and that tells you whether to use avoir or être with these verbs.
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