I'm sorry to report this but, it appears that deconstructing turrets (and other things) makes them not explode at 0hp, thus the biters will forever try to destroy them.
Savegame: http://1drv.ms/1glMK1T
[0.9.8] - Deconstruction makes things (turrets) invulnerable
Re: [0.9.8] - Deconstruction makes things (turrets) invulner
Hello, thank you for the report it is fixed for 0.9.9 or 0.10.0