Boom

I have an Accentra in my basement and always open it to scrap out the ash without any booming.

There has to be some gaseous fuel to have a boom. It won't be the pellets. If you had a really dirty burn without enough air, you MIGHT have enough flammable gases left over to cause a puff when O2 is introduced to it. I doubt the OAK has anything to do with it. That pipe should be cold due to the outside air temp and probably won't draft worth a darn so you shouldn't have reverse flow. Even if you did it shouldn't cause a pop since there isn't any fuel to make it pop. I open my insert often to scape the pot. I usually turn it down to low, wait a minute, slowly open the door and scrape away. Close the door, wait for the stove to "discover" that draft has been restored and turn it back up. I have about htirty seconds of having the door open before it goes into "shutdown due to lack of draft" mode.