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I tried to think about what the reason was that I didn't trust him.
Literal
What [cause-が原因で] him [object-を] am-not-trusting [conjectural-のだろう] [quotative-と] thought-tried.
Three embedded grammar points. (1) ~てみる ('try doing X, see what happens') attached to 考える gives 'tried to think it through.' (2) Inside the thought is the wh-question 何が原因で~ない ('what is causing [me] not to X?'), where ~が原因で is a set phrase meaning 'because of X,' 'with X as the cause' — the が marks the thing identified as the cause, not the subject. (3) That inner question is rendered reflective with ~のだろう, the plain-form conjectural/introspective version of ~のでしょう — the speaker is wondering to themselves, not asking anyone. ~と marks the whole internal thought as the content of 考える.