The simple answer is that your system tries to maintain testosterone and estradiol in relative proportions. If you inhibit the conversion of testosterone into DHT, the relative proportions of testosterone to estradiol becomes skewed and favors the conversion of testosterone to estradiol, which occurs naturally via a process called aromatization.
Even in GG's testosterone is the precursor for both estradiol and DHT. When testosterone is higher than it should be it can be reduced by one of two means: conversion to DHT by 5-alpha reductase, or conversion to estradiol by aromatase. If we block the action of 5-AR, the relative proportion of testosterone increases. In that scenario the main way to reduce the relative proportion of testosterone to estradiol is by aromatization. Thus by blocking conversion of testosterone to DHT we wind up with a net increase of estradiol.
Of course this is a simplified explanation, and the whole sex hormone balancing system is fabulously more complicated with other factors such as SHBG and the various signaling hormones (LH, GH and others) coming into play.