Why Would a Recount Happen
An automatic recount is decided on a state by state basis, unless ordered by a court.
According to numerous reports, the Presidential race will come down to only a handle of states including Wisconsin.
According to Politico, a recount would happen in Wisconsin if a candidate requests it within three days of the initial tally. The recount is free to the requesting candidate if the margin of victory is less than 0.5%, otherwise it must be paid for in advance.
Pennsylvania and Michigan are listed as other deciding states.
A recount would happen in Pennsylvania if the margin of victory is 0.5% or less. In Michigan, a recount would only happen if the vote differential is at or less than 2,000 votes.
A full list of recount criteria for every state can be found here.