Sua sponte is latin for “of one’s own will.” When referring to judging, it means of the judge’s own volition. In other words, neither of the parties have asked the judge to do something. The judge just decides that s/he is going to do it. Although there are rare circumstances where the law permits a sua sponte ruling (such as when a judge is to decide whether to self-recuse), for the most part, the law disfavors such rulings.
Why? In my mind, it’s because a sua sponte ruling is often a red flag that the judge is: 1) becoming embroiled in the litigation (overly emotional and having feelings that drive outcomes, rather than intellectual discourse); 2) favoring one side over the other; or 3) acting to protect him or herself rather than being driven by the parties’ motions, law and the facts.