Why are so many NFL teams changing their uniforms?

I was watching the Patriots @ Broncos last week, and both teams had different uniforms than 2008. The Pats have a different helmet and number style, and the Broncos, I gotta say, look ridiculous. Brown uniforms (?) with too many stripes. Now i'm watching the Broncos @ Chargers game and once again the Broncos are in these ridiculous uniforms, while thr Chargers have a different number style?! Did many NFL teams switch uniforms or are these just throwbacks/alternatives that they are wearing?