“Winning” in this context can be understood to be mean “in the lead”.
For example, let’s imagine team A and team B are playing a game of soccer. Halfway through the game team A has 3 points while team B has 2 points. In this situation, you can say “Team A is winning.” In your example, the word “winning” functions as an adjective, it describes the condition of the team at a given point in time. This is called the present participle. But the important thing to remember is that the present participle, despite its name, does not determine tense.
Consider this example:
“The water was boiling when I entered the room.”
In this sentence, “boiling” functions the way “winning” does in your example.