1.5 year old female Pyrenees mix has become aggressive toward 6 year old male Aussie

[24korange]

When we got our Birdie as a puppy, archer (the Aussie) was more dominate over Birdie. She was smaller than him and he would correct her at times but nothing crazy. Both dogs are fixed. About 9 months ago, Birdie went after archer relentlessly after he tried to correct her. Things got better up until about a month ago. Birdie went after him over a toy several times over the course of a few days. We separated them for 24 ish hours, gave no preference to either dog, got rid of all toys. We then tried to treat Birdie as the top dog. Feeding her first, leashing her first, treats for her first. Things got better again. We thought maybe she just was having a rough few days for some reason. She just went after him for apparently no reason no toys were present, she had the good “spot” while we watched tv. He simply stood up and shook off like dogs do and she snapped. We’re seemingly at a loss. Any advice is appreciated.