Why does our dog act so temperamental around this other dog?

My aunt comes over to visit us frequently because she lives close, and when we had our old dog she'd bring her little shih-tzu with her because they got along really well. Our new dog is strange. She is a big doberman. The first time she met him, she tried to attack him, and ripped a few hairs off his belly. Months later we decided to reintroduce him and she was fine, running around him playfully and wagging her tail. Yesterday we put them together again. For the first 40 minutes she was being really playful and teasing my aunt's shih-tzu with her toy rope. He was nervous around her, but he started to loosen up. Then unexpectedly, she just jumped on him aggressively. He wasn't doing anything, just sitting quietly. My dog is fine with all other dogs except my aunt's dog, and I don't understand why.