A friend of mine in his final stages of PhD left for Pakistan. His entry visa had been expired and hence he applied for a new one. The security clearance took more than 9 months but he finally got visa and arrived here. The same month he arrived, he also received his approval for Canada Immigration. So he thought he should visit Canada before starting his classes again.
However, when returning from Canada, his visa was revoked and he was denied entry to the States. I have no contact with him since then.
I was wondering on what grounds his visa could have been revoked. He had security clearance, valid entry visa and he was a regular student. Any ideas? Thanks.

