Here's the entry for Donaubauer in Hans Bahlow (trans. Edda Gentry), _German Names_:
_Donaubauer_ (freq. in Munich), also _Donabauer_: pine tree grower, cf. Hof [Farm] Donabauer in U.Aust. (1414 "vor dem Tannech," also Donach for Tannach in U.Aust., represent the dialect form; cf. Tanngraben in U.Aust.: old form Tongraben); Donhauer = Dannhauer, Tannhauer [pine cutter], also _Donheißer_ and Dannheißer = Tannhäuser; Donabam = Tannebaum [pine tree]; _Donhauser_ (freq. in Munich).
I figure the Dona to Donner change may be folk etymology at work: having lost track of the 'pine' connection, people substituted the most familiar Don- word they could think of. Or something.