Check the replay - did the ship close to DUC range, then destroy the target in a single volley before it could reach an adjacent square to board? You'd think that a ship set to capture would not actually fire on its target unless it had shields, but you never know...
/me wonders how capture will work in SE5 - will you have to close to a set range to beam over troops, or are there boarding shuttles that have to "ram" the enemy ship, or has boarding been removed like in Starfury???