。
If it rains, the match will probably be canceled.
Literal
By-chance rain [subject-が] fell-[conditional-たら] match [topic-は] cancellation [into-に] become [probably-だろう].
中止になる ('becomes cancellation' → 'is cancelled') is the になる pattern of change-of-state applied to a noun: the event is presented as transitioning into the state of being cancelled, with no explicit agent. A slightly more passive, fate-tinged framing than the verb-passive 中止される ('is cancelled by someone'), which is also common in announcements.