There was no apparent outside agency performing this miracle/conjuring trick. All by itself, the omnibus turns into a hearse before his very eyes.
The passive voice would be:
'suddenly turned into a hearse (by whom/what?). There is no other agency performing this action on the omnibus. It is the agent performing the action of this transformation.
The next step in the viewer's logic, is trying to reason how that happened. If then some viewer wants to come up with some conspiracy theory or extra=terrestrial intervention theory, then the people HE tells about what he saw will hear, "It was turned into a hearse (((by his choice of conspiratorial agent))) as a sign that we are all going to die soon." etc