I went back to 50cc just to get a star in one of the cups. During that I was hit with more blue shells than at any moment in 150cc.
I noticed this too. It makes me wonder if the size of the first place position holder's lead has anything to do with the frequency of blue shells. It's a lot easier to get a sizable lead in 50cc than it is 150cc.
Or it could just be a coincidence. If the blue shell truly appears randomly then there will be races where there are hoards of them and possibly races where there are none at all.
I'm thinking it's got something to do with the lead of the first player. I've noticed this in single player and multiplayer. If there's a guy way out front you'll see a lot more blue shells flying.
Another thing that I find odd with the AI is the fact that it seems like it'll always get quiet the last lap of a 150CC race up until I get around the last corner. I've had games where no action will happen, then I'll get blue shelled, red shelled and hit by someone with a star. I'll go from holding first place the entire race to 8th or 9th.