Something else - you can't make an AoO if your target has cover. In this case the fighter turned the table on the necromancer (dealing subdual damage) and it provided cover (see reference to archer) - hence it provided cover to the fighter - so he could not take an AoO.
PHB pg 151 "You can't execute an attack of opportunity against a creature with cover relative to you."
The necromancer never needed to make a concentration check since he didn't need to cast defensively in the first place. Ta da spell goes off.
The necromancer could also have taken a normal move (to get away from the fighter) and as long as he kept the table between him and the fighter no AoO, then he casts his spell.
PHB pg 151 "You can't execute an attack of opportunity against a creature with cover relative to you."
The necromancer never needed to make a concentration check since he didn't need to cast defensively in the first place. Ta da spell goes off.
The necromancer could also have taken a normal move (to get away from the fighter) and as long as he kept the table between him and the fighter no AoO, then he casts his spell.