You are not automatically liable for the damage: it depends on whether you were at fault which, in a case like this, generally comes down to maintenance. If the roof was in good shape, and you can show that it was (e.g. by photographs; by your own testimony) that it was, and the shingle only fell due to the bad weather with no reason to have previously thought there was a risk of this, then you should not be liable. Conversely, if the neighbors could somehow show that the roof was in bad shape--e.g. if they have other shingles which have come off your roof and landed on their property, which tends to show poor repair or condition--they could win their case.