It could also be an ancient prophecy that is misinterpreted. The Oracle might give a prophecy that the gods misunderstand as Percy being a threat. So, in an effort to prevent the prophecy from coming true in what they think is the wrong way, they disown him.
It could be an enemy that manages to sneak onto the Argo 2. Maybe a monster that they thought they had defeated before but actually just bided its time and then attacked Percy when he was least expecting it.
It could be a new enemy. Maybe a powerful and vengeful demigod who is jealous of Percy's fame and power. This demigod could have found a way to use some ancient magic or some political maneuvering within the demigod world to get Percy banished to Alaska.
It could be a new, inexperienced doctor. Maybe they make a mistake during a surgery that Meredith is involved in, accidentally causing her harm. For example, they might misread a vital sign or use the wrong instrument.
It could be a rival assassin. They might see Kassandra as a threat to their own reputation or contracts.
It could be a Sith or a Sith - inspired enemy. They are often the ones causing trouble for the Jedi, and shooting Ahsoka would be a way to strike at the Jedi Order.
There's also a possibility that it's an accident. In the chaos of a battle or a tense situation, a stray bullet from either side could hit Katniss, even if no one actually intended to harm her directly.
As I mentioned before, a member of his own wolf pack. In a pack, there can be jealousy and power struggles. If another wolf wants more influence or has a grudge against Jacob, they might be the ones to cause him harm.
It could be a mischievous Pokemon. Maybe a Ditto that has gone a bit wild and accidentally turned into a Pokeball and then somehow 'captured' Ash when he touched it.
There's a possibility that it's an old enemy who has somehow regained power or found a new way to attack. For example, Frieza's family or the Red Ribbon Army if they had some kind of resurgence. They might target Vegeta and Bulma out of revenge or to gain access to Capsule Corp's technology which Bulma controls.
A new and powerful rogue ninja could be responsible. This ninja might have a vendetta against Konoha and see Naruto as a symbol of the village, so they target him and use their strong and devious techniques to injure him badly.