It's because you did too much damage too quickly and forced the boss into it's roaming mode without disabling the shield first. You need to disable the shield to do damage to the boss.
Hey there, Appreciate you reaching out to us with this issue. So is this something you noticed taking place more than one pass through? Would you be able to clear out your [url=https://www.bungie.net/en/Help/Article/12539]cache[/url] and see if that fixes things.