Not necessarily. While those can be causes, they aren't the only causes.

It's simply a matter of harmonic vibration. Get the bike vibrating at the right frequency, and the vibrations increase, potentially to the point the bike becomes unstable.

At the right speed, with the right weight distribution, a bike in perfect mechanical condition can get the wobs.

What you have to do is alter the condition that it is causing it to vibrate at that frequency. As mentioned the most effective can be a knee into the top tube damping the vibration, loosening a death grip, altering weight distribution, all can help.

Changing speed also will usually stop it, Speeding up or slowing down. Because its a matter of vibrating at a particular frequency, speeding up can stop it, i.e. a bike that wobs at 35mph, may not wobble at 40 mph. So if it's not to severe, pedaling through it can also be an option.

Also realize that the bike is not going to shake apart, and it's rare for the wobs to be so bad that they'll cause a crash in their own right. The bigger problem is crashing by overreacting to the wobbles. So relax, and put a knee on the top tube, and relax your grip. You'll be fine.