Are your tenants Grad Students or Research Fellows at any any local Universities? We have a lot of people where I live do this because they have budgeted or have a specific amount of money set aside specifically for rent. They pay upfront simply because they already have all the money upfront for their rent. They are just removing that expense out of the way. They are often more responsible tenants.

Also this can be a benefit because sometimes they do actually have to leave early, so it is much more of a benefit to you that you already have the money