1 They experienced something negative
People Touch their noses when a negative thought comes to their minds through any of their senses. If a person hears something he didn't like, he might touch his nose (See also why people touch their faces while speaking).
2 The person remembered a negative thing
If a person is reminded of something he dislikes, for example he is asked about the low grades he got, he might feel like touching his nose.
3 The person saw something he didn't like
If a person see something unpleasant such as a scratch on his new car then he might feel like touching his nose.
4 Negativity results in contraction in blood capillaries
When a person thinks of a negative thing his blood capillaries might contract and as a result he might feel like touching his nose.
5 If he lies while believing that lying is wrong
If a person lies and at the same time believed that lying is a bad thing, he might feel like touching his nose. People who don't think that lying is bad are very likely not to touch their noses while lying.
6 Feeling uncomfortable
If a person experienced feelings that he didn't like then he might feel like touching his nose. Again the fact that those feelings were believed to be a negative thing resulted in the desire to touch the nose (See why some people fold their arms).
7 It's a built in body language gesture in humans
The act of touching the nose is done by humans all around the world and has weak connection to any specific culture.
8 Any negative conclusion the mind makes can trigger this gesture
Any negative conclusion that mind makes can force a person to touch his nose. The itching is usually very hard to resist and in most cases a person will hardly be able to prevent himself from doing it.
9 Itching for no psychological reason
A cold or any factor unrelated to psychological reasons can lead to nose touching however in most cases a psychological reason is present.