Think about it. If you stopped your action for a flashback to ‘show’ why the protagonist is doing what they are doing every single time the action changed, your story would never go anywhere.
That said, though, you should ‘show’ whenever possible, and you do so by simply going through your manuscript during that all-important second draft and zeroing in on all places where you, as the writer, simply told the readers what they needed to know.
Could that information be shown without breaking the pace of the story?
If so, then change the ‘telling’ to a flashback or a conversation between the protagonist and whomever else is nearby as a way of inserting a mini-story into the larger story you are telling. The mini-story tells the ‘why’ of what’s going on in the larger one.
And that’s how you ‘show’ instead of ‘telling.’