Having been to many hackathons, I'd say that far and away the most likely reason for this is that Java is taught in most CS curriculums. Hence students can come in and make an Android app over the weekend, whereas building an iOS app would require learning a whole new language.

I bet if you looked at winning hacks the Android skew would be much less defined (as winning hacks are usually developed by people who learn outside of class and have prior experience with the platform).