It means trouble because you go back to the past and reawaken old emotions and issues you might have been trapped into.
The moment he decides to break up, a huge part of him has already given up even if he says that he's ready to give it another try.
In most cases though, he won't even want to try.
He is already moving on with his life and maybe even focused on a new relationship.
If you stay invested in him, you keep giving hope to a ship which is sinking and he's doing nothing to rescue it.
Remember that it takes a very profound shift for a collapsed relationship to be rebuilt.
In most cases couples fail with getting back together because they don't have the time, energy and focus to follow up on their resolutions.
Saying things like
"I have changed..."
"Let's make this work..."
"We are good for each other..."
Is not enough!
You need much more than that!
You need to understand the EXACT limiting factors in your relationship + Find a set of strategies which will tackle that specific challenge.
Once you understand these dynamics and have a battle plan, you still need to take consistent action for months until these new patterns become REALLY part of you.
If he is only half into it, no matter how hard you try, you feel that you swim against a powerful current that takes you in exact opposite direction to where you want to go.
My advice is simple: focus on the future, not the past!