You are right that some people will see a first degree in engineering and wonder something like "if this applicant likes (and is good at) math enough to do grad work in it, why not do the undergrad in it too?" Where you are wrong is in thinking that answering that question will help your application at all. (The specific answer of "my parents made me" will probably hurt your application, but I will say that in my opinion there is no answer that will help.)
What will help? Sentences that rebut the worries or doubts they may have about you:
- engineers don't learn enough math to do well in grad school
- people who don't know what they want at 18 can never pursue their true dream
- people who change majors don't have the commitment and passion we need in this field
So, focus on what, as a math-loving person, you got from your undergrad work. Point to the courses you did well in, the electives you took, the projects you worked on, that helped you understand that graduate-level math was right for you. Talk about how committed and passionate you now are about math - and don't worry about whether teenage-you was committed and passionate about engineering. Admissions committees are aware that undergrad choices are made for a variety of reasons, including not knowing much about specific undergrad programs and not having much freedom of choice. Talk about your purpose now, not who you were four years ago. That four-years-younger person isn't applying to grad school; today-you is.