Taking into account the fact that Drake's reaction was completely different from what he expressed later on in his twitter feed, It's clear that Drake had already rehearsed what he would do after she did a "surprise kiss". Also, there have been previous incidents in media where things like this have been staged.
Madonna has spent decades getting her name in the news and this is just one more way to get it there. Drake looked sort of stunned after it happened. I don't think he expected a woman old enough to be his mother to give him a big, passionate unexpected smooch either. Good thing he's not pressing charges.
Yes, it was a publicity stunt. Everything that Madonna seems to do these days is a publicity stunt. This isn't the first time that she has done something for publicity. She had the kisses with Britney Spears and Christina Aguilera. That is the world in which she lives. Kissing Drake was just another way to grab our attention.
The Madonna and Drake kiss at Coachella was definitely a staged publicity stunt. This is what Madonna does best. She is an expert at self-promotion and this was a perfect opportunity to get some attention. Drake is a young, hot star so is makes sense that she would want to latch on to him and once again cause a media stir.