It's rather difficult to establish Princip's motives for going to Schiller's.

On one hand – and this is something that is very often forgotten in accounts of what happened on 28 June 1914 - when Franz Ferdinand's chauffeur turned off the Appel Quay onto Franz Josef Strasse, the route that took his limo past Schiller's and past Princip, he was not "taking a wrong turning" and so blundering entirely coincidentally into the path of the assassin. Rather, he was deviating from the revised route for the afternoon that was decided on after the initial assassination attempt went wrong, leaving the occupants of another of the cars in the motorcade injured and in need of hospital treatment. The Franz Josef Strasse route was the original one that the motorcade was supposed to follow before the Archduke took the decision to travel on to the hospital to visit the wounded instead. This certainly suggests that Princip's decision to stand among the crowds outside Schiller's was based on the expectation that the motorcade would follow the planned route and he would get a second chance to kill his target.

But, on the other hand, to presume Princip went to Schiller's on this basis presupposes that he didn't assume the motorcade would either be diverted or cancelled outright as a result of the earlier assassination attempt.

Knowing the way that security services would respond today to an assassination attempt on a major figure, it does seem incredible that anyone present in Sarajevo that day would assume events would proceed as planned, rather than that the whole day's ceremonies would be cancelled and Franz Ferdinand whisked back to Vienna as quickly as possible. But that's presentism, and we're talking about a much less sophisticated time and place.

The fact is that Princip was not alone in hanging around outside Schiller's. The evidence taken after the assassination shows that there was still quite a substantial crowd on Franz Josef Strasse, and hence the presumption is that quite a few of the people who had turned out to see the Archduke – and who knew nothing of the change of planned route to divert to the hospital - did assume the motorcade would resume and that it would follow the route that had already been announced. On that basis, I think we can be moderately sure that Princip's choice of Schiller's as a place to stand was not coincidental. Dolph Owings' translation of the transcripts of Princip's trial says as much. But Princip himself did not make this claim, and according to Vladimir Dedijer's seminal The Road to Sarajevo (p.323), he said merely:

After Cabrinovic was caught, I went to the corner of the Appel Quay and Franz Josef Street, just at Schiller's shop...

without explaining why he did so.

Sources

Vladimir Dedijer, The Road to Sarajevo (1966)

Dolph Owings, The Sarajevo Trial (1984)