Can a landlord demand the tenant pay the landlords homeowners insurance deductible for a fire whose cause is not yet determined?

Unfortunately, it depends. The ability or inability to make this demand is found within the rental agreement between the tenants and the landlord. To keep a security deposit would require that the damage (fire) was the result of acts or the failure to act on the part of the tenant. If the tenant started the fire intentionally, or through negligence, then the damage would be on his or her dime. If there is no determination that the tenant caused the damage, it would be more like a burst water pipe, just one of those things that a landlord has to put up with. Understand that the landlord wants the tenant to pay his deductible. People in hell want ice water. Without additional information concerning the terms of the rental agreement, as well as a determination from the arson investigator, any further input is impossible.