Way way back in the before time, the "T" meant "transsexual" not "transgender". At that point in time, there was a considerable amount of medical gate-keeping involved in any open transsexualism.
At that point, the T was pretty much exclusively comprised of very, very, very homosexual males - Kinsey 6 gay, if you will. In somewhat crass terms, males that were so extremely gay that they desperately wanted to be female so that they'd be even more attractive to males. To some extent, that sort of made sense to include with the LGB - they were G after all, just G with a penchant for female presentation. Of course, they also tended to present like normal average females instead of the pornified bimbo look that many of the current transgender cohort seems to gravitate to.
Currently, however, the TQWTAF+ has nothing at all in common with the LGB.