Mold arising out of a sudden and unpredictable flood would be part of the casualty loss. But the way you frame the question makes me think you have doubts yourself. It sounds like you recognize that the drainage around your house was improper, but it had not caused problems before because the weather had been dry.
I think this is something you have to decide on your own based on the circumstances. If you had a known drainage problem for years with small rainfalls but it never stayed wet long enough to damage the house, that might be seen as an ongoing problem not eligible as a casualty. But if it's been so dry that you never even had the opportunity to discover that the drainage was bad until this year, that might be a casualty. If you claim it as a casualty, and if you get audited, you will need to be able to convince the examiner of your position. It's your risk to decide to take.
