Originally it was just a few guys kneeling. If people ignored them, it would've faded away. Instead, people made a big deal about it. Naturally, it's escalated because of all the attention it brings. If the protests bother you, and you've been losing your mind over them, pat yourself on the back for causing them to grow.
Personally, I think it's silly - the notion that we can't have a sporting event without singing the anthem first. But if you claim it's a celebration of freedom, or respecting those who have fought to protect it, then guess what? People have a right to exercise that freedom in ways you may find distasteful. If you want them forcibly stopped, then you're not really about freedom - you're about your own freedom.