My immediate thought was that the shield gave a bonus to Ref. From what I understand of spells and effects, they attack PCs various defenses, so the dirt burst attacked each PC's Ref. It hit the wizard, rogue, and cleric, but missed the fighter.
The rogue was prone, and as already stated, made him easier to hit (loose dex to AC and Ref?) so while he might have typically been missed, the attack did hit him due to the circumstances.
The fighter, still on his feat, got his full Ref and got his shield as some sort of bonus (feat or fighter ability perhaps?), which made the attack miss him.
I don't see how Evasion could come into play, since the dirt AoE should be an attack (from the rules as we have seen them), and therefore rolls to see if it misses, hits, or crits. Under those options, there can be no 3e evasion ability (but there could be a Imp Evasion like ability: even if an AoE scores a hits on you, you only take half damage from the attack).