It's pretty convoluted, but the short answer is that theythought he might be a Soviet spy. .
Oppenheimer had, when younger, had some friends and acquaintanceswho were communi…sts. He had drifted apart from most of these, butwas still on fairly good terms with at least one of them. A Sovietagent approached his friend, and that friend spoke with Oppenheimerabout it. Very briefly and casually; it was something along thelines of "So-and-so was wondering if there anything you can do tomake sure the Soviets are kept in the loop, perhaps unofficially?"and Oppenheimer's response was something along the lines of "No,that's not how to do these things." The problem was that when Oppenheimer reported this to hissuperiors, he didn't report it accurately and tried to hide his owninvolvement and that of his friend. He just said that "some people"had been approached on behalf of the Soviets. So the US started abig spy hunt for these Soviet agents, and Oppenheimer's responsesto their questions weren't adding up. In fact, he wound up makingthings a lot worse for his friend than was actually justified,because the Government already suspected both Oppenheimer and hisfriend based on their (known) past association with communists, andOppenheimer trying to cover it up made it look as if theirinvolvement had been much more sinister than it really was. (MORE)