No official statement found online, but there has been several speculations presented that somewhat agree to the reason behind the TX symbol.
Sources have cited that the letter T stands Taker (a shortened version of Undertaker) and the X (others say its a cross) is something where you are hung by. Undertaker once crucified Stone Cold Steve Austin.
However, the X have some added details just not to offend, as sources have suggested, other's religion. It was thought to be designed that way to reflect the other dark names he bear such as The Lord of Darkness, The Man From the Dark Side, The Deadman and The Demon of Death Valley.
Another probable explanation claims that it stands for Texas. Undertaker, whose real name is Mark William Calaway was born in Houston, Texas on March 24, 1965. The symbol is then thought to somewhat a reflection of where he came from.
Know more about the Undertaker with these Fun Facts About WWE's Phenom, featured on BleacherReport.com