I changed the option once to not-vote after reading an article about it being better for the community if you use your votes on others, and ever since then it hasn't automatically re-checked itself, which I think is a good thing when I didn't notice an actual setting to decide what my default is for that. I'd like to think if I chose to upvote my own post (before posting) that it would keep the box checked on my next posts until I decide not to again.
No idea why it would suddenly stop being checked for you though, I just know how mine stopped being checked