His not fired by yoshiki but was ask to leave. Its either Yoshiki or Taiji to leave because both is defference in their genre of music. So its Taiji to leave because Yoshiki is the leader of X.
Apart from music difference (where taiji want X to be more heavy metal/ thrash type of band), it is also because of money.
He then joined loudness as a sessionist, and then open his own band "cloud 9" "dtr" and the latest one i forget. none of them that successfull.
anywya taiji had joined X in 1985 but quit to form the band FATIMA with JUN (ex guitarist of x ) and then join back in 1987 (not 1988) but all these is during their indies era, only hardcore fans care for their indie history.
after taiji left, X has never been the same. (seems that there's an impact of taiji parting X)