Did she just start this when you got bronchitis and started letting her out? Then I'd say it's the running outdoors that is making her hunting instinct come out. Our girl, Pookie, is well behaved indoors except for barking when she hears certain noises, but when she goes out in the backyard and runs around, she gets a lot more aggressive. Walking is probably the best discipline for them.
Interestingly, I was just reading a breeder's site and they said that the female Yorkies are more domineering than the males and tend to want to domineer when they are with other dogs. Our girl has exhibited the same behavior with other dogs. She avoids mixing with other Yorkies at the meetups and then when one gets too close, she'll get aggressive. Also females are more the hunters than male Yorkies, so they will develop fierce hunting instincts. They were bred to hunt rodents and small animals, so lacking that, yours probably goes for hunting the headlight reflections and guarding her territory against them.
Try "owning" the area of the living room she does this in. In other words, when she sits and waits for headlights, go over, put yourself between her and the wall and do not allow her to lunge or jump or get close to it. Spread your arms and legs wide and show Bella that the wall is your territory and she cannot just take ownership of it since it is yours. When she tries to get past you, block her with a spread out hand and say "no" firmly. If you have someone you could get to drive through the area that throws reflections for several minutes of this training each night for a couple of nights and put yourself between the wall and Bella, you may teach her to leave the wall alone and respect it as your territory.
If that doesn't work, look into getting an electronic dog repeller. You set it in the area you don't want them in, and a motion sensor sets off a high pitched sound when they enter the area. |