He then proceeded to drop his bombshell about getting rid of homosexuals in the military. So ADM Zumkumpft (commandant at the time) did something rare and said publically that "we won't turn our back on our people".
So the USCG was a target for what I believe are the following reasons:
1. Whatever the fucked up reason was that Trump tried to sideline the USCG in the first place, which was probably that he had no idea what the USCG actually does.
2. Now he wants revenge because the USCG stood up to him.
3. The USCG is a smaller branch of the military and doesn't have the same lobbying power as the other services.
4. He can't tolerate a woman in a position with so much power.
5. He needed to make an example of someone, and she became an easy target for all the reasons above.
So let me say it here and I'll say it again and again: the republicans are cowards. They are mentally, physically and morally weak and depraved. Trump is a man-child with no real intellect or strength trying hard to make everyone fear him because no one likes him, not even those that are closest. The people that follow him are obsequious and fatuous, choosing to be stupid enablers because they are as repulsive as he is. The rest of their leadership is cruel, treasonous, and have gotten where they are by kissing the ass of people they think are strong or have money, and trodding on those they see as weak and helpless. Damn them all.