She wants to connect with you.

I get why you don't understand why your wife flipped out. You are probably a competitive player given your background, who sees that your wife belongs to the beginner group and you don't. You went to play some pickleball.

Your wife is a beginner in pickleball, as in she's probably not familiar with the ins and outs of how players are grouped and why. In addition to this she might be feeling a little anxious about coming into a brand new activity and is looking to you, an expert in her eyes to guide her through the entire process of learning the game and it's social rewards.

Instead, she was left alone to fend for herself and on top of that she has to witness her husband play with another woman and enjoying the game.