There is a rousing discussion taking place at Christopher Johnson's MCJ over just when the Episcopal Church began its downward spiral. Johnson reasonably suggests that the institution's refusal to charge Bishop Spong with heresy 30 years ago marked the beginning of the troubles. Commenters have suggested numerous other possibilities including the heresies of a predecessor of Spong's, Bishop Pike, who also got away with them. Other suggestions include the church's liberalization of rules concerning divorced persons remarrying; the ordination of women; the church's endorsement of birth control, beginning with the cautious endorsement years ago of the use of contraception in sharply specific circumstances and culminating in our time with the joyful celebration of abortion for any reason, any time. Professor Tighe obligingly notes the remarkable number of heretics entertained by the Episcopal Church over the years, blotting her escutcheon all the way back to the nineteenth century.
Ironically, one seemingly likely culprit for inclusion in the Episcopal rogues gallery, Gene Robinson, the adulterous attention-glomming Bishop of New Hampshire, merits only a brief mention in passing by Chris Johnson and by none of his commenters. I can't help thinking Robinson might feel snubbed by that (he does so exalt in being snubbed) but the astonishing fact is Robinson's theology, such as it is, is actually less appalling than those others mentioned above, which should give anyone not familiar with the Episcopal Church a fair idea what a dreadful state she is in these days.
I think the correct answer to the question who or what led to the demise of the Episcopal Church and most of mainstream protestantism is "all of the above and lots, lots more." Lacking strong central authority (or Magisterium, if you will) left most protestant denominations utterly defenseless to assaults from the radical proponents of 1960s "counter-culture" who successfully completed their takeover of most of those institutions, including the Episcopal Church, by the end of the 1970s.
Bishop Spong repeatedly claimed his church "must change or die" if the "reforms" he flogged were not enacted. They were enacted and his church continues to die. Meanwhile, the Catholic Church, which had a mighty close call with the counter-culture, suffering egregious damage,has nevertheless managed to hold the line in her moral teachings and theology (let us all pray for the repose of the soul of that unsung hero Pope Paul VI and give thanks for his brave stance in Humanae Vitae). After a steep decline in membership during those wretched years of folk masses and crappy liturgy, not to mention the unspeakable scandal of rapist priests and their protectors, the tide has turned for the Catholic Church, thanks in large part to our present Pope who is slowly making repairs and stressing traditional teachings and worship. What a surprise: membership is on the rise again and, even more heartening, so are vocations.