It can be caused by a combo of things.
The front tire being out of balance (the red dot on a new tire should line up with the valve stem - no red dot? Get a new tire they are screwing you with an old tire) and it should have less then an ounce of weight - if more then an ounce they do not know what they are doing.
Tube wrinkled inside the tire - only applies to toobed tires.
"New" tire is actually old and hard - splains itself - buy a cheap tire and it is probably 4 years old and feels like a brick. Online tire dealers are notorious for this. Check out Bridgestone tires for the brick feel and you will see what I mean.
Low air pressure. Do NOT use the pressure rating on the tire, use the bike manual. The same tire goes on a ton of different bikes with different uses and such. A Ninja needs way more air then ours does, and they use the same tires. I use about 36 in front, 38 in rear. I have tubeless Metzlers on a SM.
Cupped tires caused by ;low air. Replace tire.
Bad rear wheel alignment. This is crucial, check it yourself to be sure. Do not trust the little marks on the arm adjusters, count the threads with a toothpick or use a laser aligner like unclecharlie showed me. I'm buying one of them, they are cool.
Bad front bearings in steering head. Rare, but could be the issue.
Bad swingarm, really rare, but the front and back must be aligned and in harmony or vibration occurs.
Mine actually wobbles now too, but it is close to replacement and it was pretty low on air and is cupped. I put in about 5 more psi then needed and it is better, but still wobbles ever so much in the 35-40 zone. Every time I replace the tire it goes away.
