I think it's because of her passive and AP conversions.
Damage dealt to towers is equal to your base AD plus either 100% of your bonus AD or 50% of your AP. AP gains are a lot higher than AD gains, so in theory both would normally deal approximately as much damage, if not a little bit higher for AP champions.
However, AD champions get a lot of attack speed to go with their AD to take towers quickly, while most AP champions don't, leaving most AD champions just burning down towers later on while AP champions take a much longer time.
Diana would break this rule because she gets so much attack speed from her passive and normally gets Nashor's Tooth as an early item, leaving her with a lot of attack speed and a lot of AP to absolutely murder towers.
That said, I don't play Diana so I can't really be sure, and I would welcome any Diana player to correct me if I'm wrong, but I think that might be what is happening, although I doubt that it would help push as hard as Tristana or Ziggs do.