With many foreclosures in the last few years, there are tons of properties that are lying empty. Scammers find these empty places, change the locks, and place an ad online or in the newspapers in the same day, representing themselves as the landlords offering the place for rent.
They meet people who are interested in renting the property at the house over the next few days and offer them great deals, proceeding to sign fake leases. Each of the victims has to pay a deposit - as is normal when renting a property - so they think nothing of it.
They only realize it is a scam when they are about to move in and discover other people doing the same - in the same house! By that time, the “landlords” are already in a different state, province, or county.