1 They remember something unpleasant
Reminding of something negative can make people touch their nose while speaking. They may also touch their nose when the other person is talking about something, and that person dislikes it. For instance, if a person is talking about the failures of the other person, that person may feel touching their nose.
2 They experienced something unpleasant
When a negative thought comes to a person's mind, they think of touching their senses, including the nose. See also Why do people touch their faces while speaking?
3 When they are lying
If a person is telling a lie to someone, and at the same time feels that lying is not a good thing, it might make them touch their nose while they are lying.
4 They see something they did not like
If a person saw something that did not please their mind and eyes, they may touch their nose.
5 Negativity results in a contraction in blood capillaries
Blood capillaries of a person are contracted when they see something unpleasant or think of a negative thing. This may make them touch their nose.
6 They have a habit of doing this
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 When they feel uncomfortable
Some people touch their nose at the time when they feel uncomfortable sitting somewhere or talking to someone. See also Why do people fold their arms while communicating with others?
8 The contrary conclusion made by the mind
Any contrary conclusion made by the mind can trigger the gesture of touching the nose. For instance, itching is very hard to resist, and they may restrain themselves from doing so in front of others. Just to prevent from scratching themselves, they may only touch their nose and control their action.