Because of the strict enforcement of alcohol and not being able to drink at their home or in a bar in public (someone rats on them) they go and drink in their car somewhere to not be seen....
It’s the same with sexual activity... not many guys are actively carrying condoms in Provo (or girls on birth control), so when things happen, it’s much more frequent for a pregnancy...
It’s the double edged sword of having the Puritan society... again doesn’t justify this and still makes these decisions stupid, but these are 18-23 year old brains we are talking about