When I first read the end of the chapter, my thought was that Quirrell hadn't arranged the incident; he had thought it was a "surprisingly good day" which suggested to me that he hadn't expected the troll.

After reading comments, I became less sure about that; someone suggested that Quirrell might have simply not intended for Harry to be at the scene and in danger. This seems plausible, but one thing still makes it difficult for me to believe it was Quirrell.

The troll had been enchanted against sunlight:

someone had enchanted the troll against sunlight before using it as a murder weapon and might also have strengthened it in other ways.

And Harry transfigured part of the troll:

Harry visualized a one-millimeter-wide cross-section through the enemy's brain, and Transfigured it into sulfuric acid.

But before, it was stated that Quirrell could not charm something that Harry had Transfigured:

Professor Quirrell could not cast spells on something Harry had Transfigured, for that would be an interaction, however slight, between their magics, but -

For Quirrell to have been behind this, I can see only two possibilities: 1) Harry can transfigure something Professor Quirrell has cast spells on, even though Professor Quirrell can't cast spells on something Harry has transfigured, or 2) Quirrell had a confederate (or imperiused lackey) cast the appropriate spells.

If you go with option 2, the increased complexity makes it less likely that it happened (though not necessarily less likely than a given alternative).