[nickff]

Because there are a number of internal actors who would have benefited from doing so. For example, if Putin had felt threatened by someone trying to overthrow him, and negotiate an end to sanctions (and the Ukraine war), it might make sense to sabotage Nordstream and reduce the potential financial benefits from the coup.

In short, while it might not make sense for "Russia to blow up its own pipeline", it would make sense for 'certain Russians to blow up a pipeline owned by other Russians'.