It is likely that you added enough cold briquettes, that they intially threw off some poor smoke. You want to manage your fire so that you are never adding a lot of additional cold fuel mid-cook. Adding a little bit, more often is best.
I'm feeling bearish, and I'm packing a Wusthof Grand Brisket slicer from MABA
Whip It Off, Chambers!
"perhaps...but then again...maybe not..."