1 They heard something negative
In body language, a person might touch their ears if they heard something that they didn't want to hear. Hearing negative news, for example, could motivate a person to touch their ears.
2 They said something they don't like
If a person said something that they didn't like then they might touch their ears as they hear themselves talking.
3 They are lying
If a person started lying and they didn't like that or if they didn't like how they told the lie then they might touch their ears while speaking. If the person is okay with lying, this gesture won't happen.
4 They don't want to hear that
A person might touch their ear if they don't want to hear what is being said. This gesture might be the adult version of putting both hands on both ears. (See Why do people touch their nose while speaking?)
5 They disagree with you
If a person disagrees with you on something that you just said then they might touch one of their ears right after you say that thing.
6 It might signal lack of interest
If the person doesn't want to hear what someone is saying then this might show lack of interest. The gesture itself does not mean that a person lacks interest, but it means that they don't want to listen. (See Why some people are not interested in others?)
7 They thought of a negative thing
If a person thought of a negative thing in their head then they might touch their ears. In such a case, the thought represented a talking person who said something negative.