Yang Jun was from Huayin in Hongnong Commandery (弘農, roughly modern Sanmenxia, Henan).
As she neared death in 274, she feared that whoever would be empress next would endanger the crown prince status of her developmentally disabled son, Sima Zhong.
In 291, after Sima Wei returned to Luoyang from his defense post (Jing Province (荊州, modern Hubei and Hunan)) with his troops, a coup went into progress.
Empress Jia, who had her husband easily under her control, had him issue an edict declaring that Yang Jun had committed crimes and should be removed from his posts.
Quickly, it became clear that Yang was in trouble, particularly after he declined his strategists' suggestion to burn the palace gate to temporarily halt the advance of the imperial guards.