The tree actually does answer this right after revealing it's origins saying it left itself die as a sign to hopefully teach Ail and An to have compassion.
We can only theorize that it got depressed over the years and remained silent/numb over the atrocities it witnessed Ail and An do.
It's interesting that it's after having sucked
Usagi's energy that it went berserk.
What I wanna know is why it's called the Demon Tree when it seems to be a benevolent one who knows about love and warmth.
It mentions they visited several planets, did the survivors wreak havoc over there and this is what got it this name?
Fiore is definitely from the same species so if we take into account that in the timeline of the R movie (that if I'm not mistaken cannot be inserted into the 90s anime and hence is non-canon), did Fiore wander off and was stranded into the universe?
That one guy who went out for a stroll with his dog, "What the heck?! What happened to my luxury apartment? I already paid 2 months advance with deposit?!!
I WANT MY MONEY BACK!!!!"