any strong androgen reduces SHBG. also since estrogen increases SHBG, lowering estrogen reduces it also.
DHT binds preferentially to SHBG vs. T, which binds preferentially vs. E. this is why high E levels wack your free T so badly. aging men's total T does not typically fall that much, but their free T drops substantially do to higher E (and thus SHBG) levels.