This is pure speculation. In carefully perusing all sources for this case I have not seen one official mention of the door being broken down or that any door was ajar. None. Please provide a link.The bedroom door did have to be broken down. It was locked. That was the quickest way to get in at that time.
As for the back door to the house, I believe it has been posted, on this thread somewhere, that the back door was either ajar, or that someone had a code to get in. I have never seen that anywhere in MSM, so can't confirm it.
There was the 911 call where one of their friends said they can't get in and told the 911 operator they didn't know if they should break the door down etc. We have no idea if they did.
There is also the followup 911 call that states they see Spencer laying next to his bed in a pool of blood, but they can't get any closer to see more. We have no idea or no concrete source as to where they were or how they saw this. There's been speculation of a ladder, the friend having access to the house, etc. Nothing confirmed or cited at all.
These are genuinely curious and key components of this case that have not been confirmed yet or elaborated.