One reason could be a misunderstanding. Maybe someone in the pack misinterprets Stiles' actions as being against the pack's interests. For example, if he's involved in something that looks like it's endangering the pack but in reality he's trying to protect it.
One possible reason could be a misunderstanding. Maybe there was a miscommunication during a mission, and the other members of Team Natsu thought Lucy made a big mistake. For example, if she accidentally led the enemy to their hiding place. Another reason might be jealousy. If Lucy's celestial spirit magic started to gain too much attention and overshadowed the others in some way, they could have decided to kick her out. However, this is all just speculation based on the idea of this fanfiction scenario.
One possible reason could be a misunderstanding. Maybe someone framed Natsu for a crime he didn't commit, like stealing a powerful magical item from the guild. And the guild, without proper investigation, decided to kick him out. Another reason might be his recklessness. If his actions put the guild in great danger too many times, the guild masters could decide that he's a liability and kick him out.
It could be Vegeta. Vegeta might kick Goku out of a training session because he feels Goku is showing off or not taking the training seriously enough. Vegeta has always been competitive with Goku and this could be his way of asserting his dominance.
Lucy might be heartbroken. After all, Team Natsu was like a family to her. She could initially go into a state of shock and disbelief. Maybe she would try to plead with the team to let her back in, explaining that whatever they thought she did was a misunderstanding.
Natsu would probably be very angry at first. He loves Fairy Tail like a family, so being kicked out would feel like a huge betrayal. He might go on a rampage, shouting and arguing with the guild members who made the decision.
One reason could be if he was wrongly accused of stealing something important from the camp. Maybe a powerful magical item goes missing and Percy is the prime suspect because of his past issues with authority. Another reason might be if he accidentally endangers the camp in some way during a mission or training exercise. For example, if he unknowingly leads some dangerous monsters back to the camp.
Stiles might be initially shocked. He would probably feel betrayed since he thought of the pack as his family. He could go into a state of denial at first, not believing that they would actually kick him out.
One possible reason could be that he's framed for a crime he didn't commit. Maybe someone planted false evidence to make it seem like he was endangering the guild. Another reason might be that his power got out of control and the guild, fearing for the safety of its members, made the tough decision to kick him out.
It's hard to say exactly. It could be related to a dare, a competition gone awry, or even a misunderstanding among friends. Maybe the guy being lifted moved unexpectedly, causing the lifter to lose their grip.