> Imfura had his little group moving constantly, an understandable method to avoid other groups and the possible transfer of his females.
> [emphasis added] But this apparently weakened their trust,
> which is essential for social cohesion.
How do the researchers know their trust was weakened by constant moving?
You emphasised the key word yourself, "apparently". And the proof is in the pudding, they left. Now the researchers didn't ask the females obviously, but they would have a lot of experience with the behaviour of the gorillas and know that the females don't like instability and unsureness of the leadership (hinted in the article), so the obvious conclusion is that the instability of moving around caused the females to loose trust.
Note I didn't ask the researchers either and am only deducing this from the text.