I had this problem in the last few day more than enough. I have no idea why it didn't happen before o.O The main problem seems to be that the "big" decouplers (the one that you are using) are too weak to push the Boosters away, the boosters will turn to the core and "eat" the middle stage. To solve that i used the decouplers for the smaller parts, works the same but they are a little further away from the middle stage. If that doesnt work, there is also a mod with 5 sec boosters to mount horizontal on the boosters. they will push them away from your middle stage. i use them quite often for bigger rockets, hate it to get hit by my own debris xD hope i could help you