FAQ for
Primer (2004)

According to the director, the audience doesn't know. There is no information in the film that would answer this question conclusively. This is intentional, since the audience is watching from Abe's perspective, and Abe himself doesn't know. Abe can only speculate why either him or Aaron would mention the time machine to Granger in the future, and why Granger would want to come back. This uncertaintly and paranoia is what drives Abe to use the failsafe to go back and perhaps not mention the time machine to Aaron the second time around, to avoid the Granger incident from happening.

There is some hint at the end of the film that Granger's daughter might have been shot by her ex and either Abe or Aaron told him about the machine to allow him to go back and save her.