Assassins vs Backline

I recently played a match and had a row of champions completely against the back line, tanks in front. My ranged were in the middle, with tankier melee on the outer hexes to guard my ranged from any assassins. When the round started, my opponent's assassins jumped to the furthest champions (as expected), but instead of engaging the assassins, both of my melees (non-berserker) proceeded moving forward towards the enemy front line. And the assassins went straight for my (supposedly guarded) ranged champs. Any ideas how/why this would happen? Is this a bug?