ことになった

A non-past verb + ことになった can be used to express that "a situation has changed due to some external force (e.g., a decision was made by someone other than the speaker)." It's important to remember that the speaker did not make the decision that caused the situation to change. Let's compare a couple other similar grammar points to distinguish them.

ことにする - The speaker or subject made a decision to do something.

ことになっている - Something has been decided and the result of that decision is still in effect.

 ことになった - Something was decided in general, and perhaps associated with a single action.

日本にあるオフィスで働くことになったので、日本に引っ越すことになりました。
It's been decided that I'm going to move to Japan since I'll be working at the office there.

来学期田中先生は英語の文学を教えることになった。
It turns out that Tanaka-sensei will teach English literature next semester.

環境をよくするため、来月からリサイクルキャンペーンをすることになった。
The help improve the environment, it's been decided that we'll have a recycling campaign next month.

来週化学のクラスに微生物を勉強することになった。
It turns out that we're going to study microorganisms in science class next week.