Road bike very twitchy when standing in the pedals? First time test riding a road bike yesterday after years on ATBs. Loved it.

But when powering up a hill I'm used to standing up. So did that instinctively, and immediately found I had to sit back down because the steering became like a unicycle - guessing my CofG was a way forwards of the steering axis.

Is this that 


the specific bike I was riding is the wrong size for me?
I just can't do that on road bikes?
I can do that, but need to learn a different position to what works on an ATB?
Riding out of the saddle is a totally valid way to ride, you may just need to practice more. I remember having much the same reaction to my first road bike.

Almost all road bikes are twitchier/have faster steering than mountain bikes.

Mountain bikes are typically designed to be very stable to stop ruts, roots, and rocks from throwing you into a ditch or off of a cliff.

Road bikes are designed for nice flat, smooth tarmac and don't really have to worry about these obstacles.

Even within the road bike gamut, you'll find that the stability can vary wildly. A bike for touring will be very stable but one designed for criteriums will likely be the twitchiest of the lot.

I can't comment on the sizing, if you're test riding at a shop, why not try something bigger or smaller?