is considered to be the subject marker in Japanese. This means that it marks someone or something that is performing an action, or it identifies someone or something as having a specific feature (this is usually a feature that the listener is not aware of).
My father works.
My father is tall.
In both of these sentences, が
puts focus on the person as being ‘the one’ who is performing an action/existing in some way (as opposed to any other person). Because of this, が
is usually said to highlight what comes before it, while は
highlights what comes after it.
always shows the ‘target’ of a statement, when other targets also exist. This is why が
is used to describe situations that are happening ‘in the moment’.
Because the event in this sentence is only one short moment out of many possible ‘points in time’ that could be described, が
will be used.
が is not usually used to describe something about yourself, unless you are intentionally describing yourself compared to other people. If you want to talk about yourself ‘in general terms’, は will sound more natural (there are exceptions).