Here is my understanding of "red flag laws". If someone that you may or may not know deems that your gun is a threat to them or anybody else, they can file a complaint. It is then okay for the government to come into your home unannounced and seize all of your guns. You must, then, go to court, which can take up to two years to get a hearing, and prove that you are not a threat to yourself or others. In the meantime, your second amendment rights are taken away from you.
Here's the first scenario that I thought of. Say that I'm walking through a mall. I have a carry permit and I carry a weapon every day. I keep it as concealed as I can, but sometimes it is visible. If some bleeding heart liberal sees my weapon, she can say that she felt scared and threatened by the appearance of a gun, she can report me and I could lose my guns and my rights for as long as two years and maybe permanently.
Here's the second, and more ominous, scenario. They are now calling for a nation-wide firearm registry. This will allow the government to know where everyone's firearms are. Then, if the government uses the federal "red flag" law that some congressmen are pushing for, they could say that ALL gun owners are now a threat and confiscate everyone's guns.
Some people think that I'm just a "conspiracy nut". But think about the second scenario. That doesn't sound very far-fetched.