I really don't see any reason for South Tyrol not to rejoin Austria.
For one, I don't think that Austria will be prepared to give them the same generous benefits as Italy does. Also, South Tyrol has a substantial Italian and Ladin, which would be strongly against South Tyrol joining Austria, which could cause substantial problems. Note that the Italians are actually a majority in Bozen, the capital of South Tyrol and which is quite close to the southern border of the province. The South Tyrolians might find themselves without their capital if they attempt this...