Some of the eyewitnesses feared reprisals from their own community if they provided testimony contradicting the community-approved version. In other words, peer pressure became a powerful influence on the "eyewitnesses" to say the same thing and thus maintain the unity of the group in the face of contradictory evidence.
The similar emphasis on unity in the Fourth Gospel is consistent with a later date for its production, during which time community identity is becoming self-conscious, in part because of attacks on it by outsiders who did not believe, the unique vignette at the end featuring Doubting Thomas being of particular rhetorical significance in response.
There shall be one fold, and one shepherd. -- John 10:16
Holy Father, keep through thine own name those whom thou hast given me, that they may be one, as we are. ... That they all may be one; as thou, Father, art in me, and I in thee, that they also may be one in us: that the world may believe that thou hast sent me. And the glory which thou gavest me I have given them; that they may be one, even as we are one: I in them, and thou in me, that they may be made perfect in one; and that the world may know that thou hast sent me, and hast loved them, as thou hast loved me. -- John 17:11, 21ff.
The witnesses to the Brown shooting, discussed here:
The Department of Justice investigation into the shooting determined that witnesses who corroborated Wilson's account were credible while those who contradicted his account were not. The witnesses that claimed Brown was surrendering or did not move toward Wilson were not credible; the report stated that their claims were inconsistent with the physical evidence, other witness statements, and in some cases prior statements from the same witness. No witness statements that pointed to Wilson's guilt were determined to be credible. Twenty-four statements were determined to lack any credibility, while eight which were found credible corroborated Wilson's account. Nine did not completely contradict nor corroborate Wilson's account. Several witnesses reported fear of reprisals from the community for providing evidence that corroborated Wilson's account.