This, as so many things, comes under the heading of YDMV ("Your Diabetes May Vary"), a shorthand way of reminding ourselves that every physiology is individual and responds to things in individual ways, whether those things are exercise, nutrition, stress, or something else.
Point being, you probably can't prove with utter 100% certainty which component of that yoghurt is having the effect. There are, however, ways to narrow it down and identify likely causes.
First, you mention that you ate other things in the same meal. Try eating the yoghurt by itself with nothing else and see whether you get the same kind of spike. That will eliminate a whole host of possibilities.
If, based on the above test, the yoghurt does indeed seem to be the culprit, then ask yourself some other questions, i.e., do protein and/or fat tend to cause you to spike dramatically? For some people they do, for others, not. If not, then it becomes more likely that the particular carbs in that yoghurt are ones you're especially sensitive to. (That's another possible source of variation—some people have very different reactions to different types and sources of carbohydrate; others don't. For me, a carb is a carb is a carb, no matter where it comes from. That's not my opinion, but my meter's. But for some folks, it can make a real difference. You want to know which category you are in, if you don't already.) Everyone is different.