Watering flowerbed led to water damage to neighbor

[Ash Liu]

Hello

My ex-landlord is threatening to sue me for water damage I caused due to watering a flower bed in the garden. How should I defend myself?

I rented a house townhouse that had a flowerbed built along a neighbor's wall. The flowerbed had an irrigation system builtin, but the automation system has been broken before we moved in. The landlord first threatened to deduct money from our deposit if the short bushes in the flowerbed died, so I started watering the flowerbed by manually turning on the irrigation system and turning it off ten or twenty minutes later. I watered every other day in the evenings. The flower bed had weeping holes for excess watering; the water would drain out and leak into the drainage system in the middle of the garden. 

The landlord contacted me one day saying the neighbor's kitchen was flooded two weeks ago (1 inch water on the floor). The flooding occurred early morning. There's no evidence that I overwatered, and the flooding only happened once albeit I kept watering before and after my landlord contacted me. However the neighbor was adamant about bringing my landlord to small claims court, and thus in return, the landlord wants to sue me for the damages the neighbor is claiming. 

Does my landlord have a case? How should I defend myself? Present my water bill? show that the landlord knows about the broken irrigation system?

Many thanks in advance!