What are you going to do next?
What are you going to do next? What's your default next action? When you finish that email or someone interrupts you, or you finish a thought or end a conversation? What about making (or waiting) for coffee? How about when you finish thinking about something?
As soon as you finish one thought your mind's looking for the next one. It's seeking things to think about.
That's why many people will reach for their phone to fill the mental gap. Most of the time this is a default reaction, it's not because they're reacting to a new notification.
If you're happy with that and where it takes you then of course that's ok. If you're not you might want to consider changing your default.
(Another alternative is to default to whatever's shouting at you for attention latest and loudest.)
This is a habit like any other. Breaking it or reducing the impact can be as simple as moving your phone out of reach. Even better if you have to open a drawer or get up to pick up your phone again. Bonus points for turning off all but your essential notifications.
Do you remember when the only time your phone made a noise was when someone called you? And the only time that you picked it up was when you wanted to speak to someone?