Sections of the turret's physical armour is being _removed_... to convey that the turret is now _more durable_ to attacks.
Note that "gets harder to kill" is already a counter-intuitive mechanic for a structure to have. That is to say, humans _know_ that a crumbling structure is weaker - and the game even reinforces this state-of-mind since all other turrets crumble as their health is reduced. Your VFX artists needed to come up with something that visually communicates the opposite of what player's know:
* To communicate that the turret is getting stronger.
* To allow players to see the difference between a durable turret and a weak turret,
In my opinion, the current implementation is backwards. The shield should materialize (as its health is reduced) to convey that the turret is now tankier. The shield should radiate/glow when Bulwark is active.
Sure, hindsight would ask "why the turret didn't just put the shield up from the start" and maybe that's why you opt'd for the current visuals - but realism isn't important versus game clarity.
Besides, the game already shows that structures have telekinetic self-assembly magic in their initial spawn animations (and inhibitor respawn animations) - so a magically materializing shield wouldn't exactly be unreasonable.