He probably got tired of forcibly working for other people and ran away from Bishop between S5 and BTTS. Or he just got mad that Bishop revived him in S4 and decided that he didn't want to work with him anymore.
Bishop in FF says he convinced Stockman to help him. Interesting to know how he did it.
Btw, I really like how he acts in BTTS: chill, confident. Feels like he matured.
*just like with few other videos, scenes from 1x03 are edited to avoid copyright