Seems there are not really more information about what caused the accident and why the tram crashed into the bus from behind, but it happened on the straight road reserved only for trams and buses so there is no reason why tram should crash there into bus unless it suddenly stopped. Also after the crash the bus continued driving for few meters instead of stopping immediately. I've found only video after crash in this article[1]:

"The bus reportedly had only one regular passenger, but several Västtrafik employees were on board for the inaugural trip. It will now be driven back to the depot to be inspected and possibly repaired."

[1] https://www.svt.se/nyheter/lokalt/vast/sjalvkorande-buss-i-k...

I checked also paywalled Swedish newspaper Goteborg Posten (thanks to my library in PressReader), but nothing else really, just some opinion of tram union guy since tram hit the bus:

"The question is whether, for example, they’ve entered the correct right-of-way rules for trams? All vehicles are required to yield to trams, with a few exceptions, and that could be what’s “triggering” the system, he says.

– We can’t change our rules just because a vehicle like that is coming. It’s the other vehicles—the buses—that have to adapt and take into account that trams don’t follow the same rules as they do,” Sandholm continues.

He can’t say for certain how the accident happened, but he finds it remarkable that it occurred on Aschebergsgatan, a straight stretch of road with lanes reserved exclusively for buses and trams.

“I think that suggests no proper risk analysis was conducted regarding what happens when, for example, a self-driving bus suddenly stops on a tram route. And such a risk analysis must, of course, be conducted in collaboration with the tram drivers who operate on that route,” he says.

“Yes, it’s clear that this is something that needs to be addressed immediately, so that’s good. But sending a vehicle out into traffic that could come to a sudden stop on a route where no other traffic is supposed to be moving... Well, this shouldn’t have happened,” says Magnus Sandholm."

Self-Driving bus in Sweden crashes with tram on first day of passenger service