In your study, the other team's defense was a confounding variable because:

I believe the answer is It could affect your study participants' shooting performance

confounding variable refers to the outside influence that could influence the independent variable.

In this case, shooting result is the dependent variable and shooting skills are the independent variable. Beut beside these two, external factors such as opposition's defense could prevent good shooting result regardless the shooter's slkills

The other team's defense is an external influence both the dependent and independent variables that can change course of performance of the team’s performance. So it is confounding variable because it can affect the participants' shooting performance. The team's defense can work against the other leading to its loss.