Another idea and not particularly my opinion, but could it also be possible that Yoshiki won't say the actual reason bcs it would show him (Yoshiki) in a bad light? (I'm not sure if this is correct English, since I'm not a native speaker). Or maybe Yoshiki also is thinking that the reason was a bit childish and he doesn't want to talk about it anymore?
it might very well be that both reasons are one. Maybe a "childish reason that would also have put him in a bad light". What's certain is that almost everybody in their entourage knew that the break up was imminent, and Yoshiki spread the 'rumor' soon to be confirmed... I recall that paragraph from Taiji's autobiography where he speaks about his future without X in those terms... " My departure from X was inevitable. When I think about it now, wasn't everyone saying things like that when I wasn't around?"
He only sort of retroactively realized he was destined to get fired, but when Yoshiki brought it up with him, Taiji's first reaction was still "wtf are you saying man," which means whatever he did to get fired, at that specific time he didn't realize that was a huge issue, so that might rule out some wild guesses out there...
I sort of get the feeling that the ultimate trigger to him getting fired must have happened really quickly, cuz timeline-wise, those larger issues like money, contracts or whatever could still wait, but when Yoshiki fired Taiji they literally was a few days away from the three-day Tokyo Dome concerts, and Yoshiki wasn't even thinking about this huge thing when he decided to let Taiji go. They had to literally negotiate (with staff) before Taiji at least had a chance to do the Tokyo Dome concerts to exit X properly. If it's about money & album copyright, Yoshiki could honestly just wait till Tokyo Dome's over before firing him. If the central issue wasn't an urgent thing, why such a hurry then?
Besides all kinds of crazy rumors out there (i'm pretty sure "underage groupie" wasn't the issue, even if there was any), there are two more likely hypotheses that were not brought up. 1. Taiji got too close with Loudness and when Yamashita quit, he probably promised to be involved with their next album without consulting Yoshiki. X members and staff were supposedly already unhappy with him making friends with Loudness. 2. Remember all X members spent half a year in LA under stressful recording conditions and lived in the same apartment for a while, and if drugs were involved at some point, it's not like Taiji could hide it from the others, and he would probably not be the only one doing it. However, it was likely that either he was talking too much once everybody got back to Japan, or he wasn't stopping the drugs when all others had stopped.