Is there a difference between (a) 'leave somebody wondering' and (b) 'make somebody wonder'? If so, what difference?
Is there some difference in aspect?
For example, does 'leave somebody wondering' somehow imply that the wondering happens after a certain action, while 'make somebody wonder' implies that the wondering happens during a certain action?
I would not say that there is a difference in aspect per se, but the phrases would be used in different situations. The more common phrase would be "make someone wonder" which, as FumbleFingers phrased very well, would "set off a train of thought". I think most commonly one would say: "It makes me wonder [what he has been up to exactly]."
To "leave someone wondering" is a less common idiomatic expression which, to me anyway, implies that a person has intentionally left a situation or relationship ambiguous and that the ambiguity will, most likely, never be resolved: "I thought at first that we were hitting it off, but she left me wondering." (To use the other phrase: "It makes me wonder how things really stand between us.") I suppose it could refer to unintentional ambiguity as well, but in that case, I would more likely say "left me confused" or "left me completely lost". I hope this answer didn't leave you confused.