The deaths of the Wild Zords in the finale. Yes, previous seasons had their Zords destroyed in the final battle, but this time the Zords were living creatures who were being brutally killed by Master Org. Them all coming back to life in the end helps soften the blow.
Out of all the Ranger teams, this one is the least likely to be reassembled, as Shayla took their morphers, their jackets, and all the Wild Zords onto the Animarium before departing to the sky again, implying that she will be returning to her slumber until a future generation of Wild Force Rangers is called.
Out of Canon.
Cole's actor, Ricardo Medina, killed an acquaintance, Joshua Sutter, in February 2015. Reports indicate it was accidental or self-defense— Medina himself called the police, waited for them to arrive, attempted to administer first aid, went into custody quietly, and was released pending investigation. Whatever the circumstances, it's more than a little shocking to the fandom. Several Ranger actors have commented, most attempting to be respectful of both Medina and Sutter, with Jason Faunt (Wes from Power Rangers Time Force) doing a short TV interview for local news, saying that Medina is a gentle person.