Tsukishima's back story has a lot to do with family influence. His brother's success in volleyball was a big factor. He was in his brother's shadow for a long time. As a result, he was hesitant about volleyball. But as he grew in the Karasuno team, he started to find his own identity in the sport, separate from his brother.
One of my favorite moments is when Yamaguchi stands up for Tsukishima in a fanfic. It shows his loyalty and how much he values their friendship. Another great moment is when Tsukishima gives Yamaguchi some really sincere advice about his volleyball skills. It's a moment that shows he cares about Yamaguchi's growth.
In Haikyuu fanfic, Tsukishima and Yamaguchi have a very special relationship. Yamaguchi is often seen as Tsukishima's loyal friend. He looks up to Tsukishima and is always there to support him. Their relationship is a complex mix of friendship, with Yamaguchi being the more emotional and openly caring one while Tsukishima can be a bit more aloof on the surface. This dynamic makes for interesting storylines in fanfics as they grow together both on and off the volleyball court.
One possible reason could be that Tsukishima is under some sort of mind control or influence from a more powerful and malevolent force. Maybe he has been deceived into believing that Ichigo is a threat to something he holds dear.
Ichigo might initially be in shock. He could be unable to believe that Tsukishima, who might have been a friend or at least an acquaintance, has turned on him.
They might be really surprised. Some characters could be amused by the creative scenarios fans come up with. For example, Hinata might be excited to see himself in all kinds of wild adventures in the fanfictions.
I have no definite answer. Sometimes manga series end when the story feels complete to the creator or when publishing circumstances change. So, we'll just have to wait and see.