Maybe they should clarify it with something along the lines of "If this player becomes downed from a tackle attempt made by an opposing player..."
Regardless of which player is rolling the dice, I consider both players to be in the act of trying to tackle each other. I feel like the X represents the defending player getting the better of the attacker, not the attacker slipping on a proverbial banana peel and falling down. Themeatically, I can see the Minotaur's ability working when the Minotaur rolls XX; the Minotaur is so aggressive that, even when he's dragged down on offense, he makes a last-ditch surge to tackle his opponent.
For the time being we'll play it to mean that it's only when the Minotaur is having dice rolled against it but I'd be interested to know what the designer's intent was.
It doesn't need to be clarified though, it's fairly obvious because the X would mean the minotaur tackled himself and thus he could use his "standing starpower" against himself to injure himself... although why anyone would choose to do this is beyond me.
I get what you're saying, but theme and mechanics don't mix like that lest they open up issues like you had. In terms of rules, he tackled himself. For all intents and purposes when you come across an issue and the "theme" suggests one things while the mechanics something else, go with the mechanics, they where made that way for a reason.