I came across the following sentence
If A team is the winning team...
Why is the に particle being used in
Why would it not be
Aチームは in this case?
I have not come across an explanation on this usage of
First, your translation of the example is not correct, but that doesn't matter with respect to what you want to know. The example means "if there is a team that beats Team A, then...".
The verb 勝つ has the valency が―に, whereby が marks the subject, and に the object:
subjectが objectに 勝つ
The antonym of 勝つ is 負ける, which also has a が―に valency.
One famous explanation of verbs like these can be found Shigeru Miyagawa's Structure and Case Marking in Japanese (Syntax and Semantics vol. 22).