Who can say what the motivation can be and while there can never be any justification, hate crime towards Muslims and particularly against Muslim women have risen in Austria.
Reported Islamophobic incidents in Austria rose to 540 last year.
Source - Documentation and Consultancy Centre for Islamophobia and Anti-Muslim Racism
In addition and for all I know there could be other alleged reasons, supposedly there was a link between Austria and the mosque attacks in New Zealand.
Once again, to reiterate for the hard of understanding I do not believe there can ever be any justification for murder, attempted murder, wounding or terrorist attacks.