The endorsement isn't odd, what is odd to me is highlighting her pro-choice stance in Montana. There is a way for Hillary to make the endorsement and rally the base for Williams without emphasizing an issue that a majority of the state feels differently on and will harm her in the general election.
Is Montana really that socially conservative?
I didn't find a poll on abortion in Montana so was just guessing on that. The state definitely has a libertarian streak, but I still don't see it being a successful issue to run on there, similar to Wendy Davis running on it in Texas in 2014.
I imagine that while Montanans may support abortion freedoms, they aren't rabid #ShoutYourAbortion SJW types.