“Under what circumstances was Brennan receiving classified information?”
My understand is that he wasn’t. Having a security clearance isn’t the same as actually having access to classified information.
It just means that if the need should arise, the paperwork for granting access would be minimal.
Ex officials from prior administration’s are sometimes called upon to advise or assist the current administration, so they often retain their security clearance so that they can be quickly brought in the loop. If they did not retain their clearance it could take weeks or months to clear them.
I doubt anyone in the Trump administration would ask Brennan for advice, so it’s good to revoke his clearance.
It’s also disgusting that ex officials like Brennan, who are hostile toward Trump, can profit from their security clearance by appearing as paid guests on the fake news and “drop” the fact of their security clearance to make themselves seem more important.
Now, when Brennan goes on talk shows, there should be some doubt in the minds of the audience whether he has real classified intelligence or is making it up. Should be. But they will try to make it look like losing the clearance is a badge of honor.