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.
Well, one possible plot twist could be that Tsukishima's brainwashing of Ichigo is part of a larger plan by a hidden villain. Maybe this villain wants to use Ichigo's power for some evil purpose. Tsukishima might be forced to do this brainwashing against his will in some way.
Rukia would probably be very suspicious right away. She knows Ichigo well and would notice the strange changes in his behavior. She might try to confront him and figure out what's wrong.
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 common theme is growth. Both Tsukishima and the reader often grow as individuals as they interact. For example, Tsukishima might become more open and less aloof. Another theme is discovery. The reader discovers the real Tsukishima behind his cold exterior, and Tsukishima discovers new feelings within himself.
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.