“why did Trump cave?”
Trump fired a dumbass before he caused more problems.
Flynn called the Russian ambassador using an unsecured line on the day that Obama enacted the new sanctions. Calling this dumb is an understatement. Flynn is a moron.
Trump could have waited to fire Flynn later but that would make Pence look weak. Better to fire Flynn now and get past it. Next week this will be ancient history.
How do you know MF used an unscured line? Did you read it in the news?