why sometimes touching old flickering tube lights starts them properly

I've found this to be the case, too. Generally, my shop lights will flicker when turning on, especially when it is colder outside .

There are two basic phases to this kind of light bulb: a start-up phase, and an operating phase. The start-up requires more voltage, because you are initiating the plasma stream between the terminals of the bulb.

So, these bulbs are filled with gas, generally mercury vapor. When you turn it on, you're really creating an electrical arc at the end of the bulb. This arc ionizes the gas, and that is the flickering you see. Eventually, the entire length of the bulb can arc, and a plasma stream is created (plasma is just a gas of free-moving ions and electrons).

When it is cold, or if the starter arcs are wearing out (and can't produce sufficient voltage), then the flickering lasts longer. By touching the bulb, you actually provide a shorter path for the plasma stream! In effect, you get two short plasma streams, each terminating at your touch point. Then, once the plasma streams interact, and since the voltage required to maintain the stream is less than the voltage needed to establish it, the streams merge into one and the lamp will light.

This is similar to some "glowing orbs" you see in gift shops or museums... a glowing plasma (looks like lightning inside a clear spherical shell) will arc and bend toward your touch, since you are providing your own set of electric and magnetic fields.