Barbara was excited to go on a solo patrol without Batman and Robin.
She felt proud to earn Batman's trust.
She overheard a nasty group of men talking, and distinctly heard them mention a kidnapping.
She followed them several cars behind as they drove to an old warehouse. She climbed in through an upper story window and remained out of sight while listening from a dark corner, soon a van pulled up and several more men came out dragging a pair of hostages.
There were too many men for her to take on alone, so she signaled Batman to meet her.
After tossing down a few flash grenades they swooped down and took out the ten men including a minor mob boss. Batman congratulated Batgirl on a job well done.
Barbara was elated by her success but then tripped twisting her ankle. Though she resisted, Batman supported her as she limped out. She was grateful and reached up to kiss him quickly. It felt like lightning and electricity had hit her at that exact moment.
The next day a photo of the impromptu kiss hit the paper, it was a damn camera, not lightning!
She wanted another taste the next day, and secretly planned to get Bruce alone.
Bruce had a visitor that morning, Selina Kyle just happened to come over and she and Bruce were in a locked room for a good half hour.
Selina never did anything without a plan, she needed to quash anything between Batman and Batgirl.
Barbara had been waiting in the library for him for a long time, and while wandering the house overheard lots of panting and moaning from the locked room, and left in a huff.
That awful bitch! This was pure spite and malice, this meant war!