Should I use hypophora in my SOP?
I can't speak for all disciplines but in biomedical science this is generally considered an inefficient way of writing because the same sentence can be simplified to


  My reason(s) of shifting to biology from physics is(are)...


It's also considered tacky but I probably have to attribute that to overuse and misuse. I will only use hypophora if the statement is highly unusual or seemingly illogical. This is a bad use because the statement is not dramatic:


  To make tea I first need to bring the water to boiling. Why? Because...


Contrasting to:


  To make tea I first need to bring the water to boiling with a silver spoon in the kettle. Why? Because...


is probably synchronizing better with the readers' mental rhythm: "Huh? Why the heck would you put a silver spoon in it?" Then the following "Why?" would serve a better role: to resonate with the readers.

Surely enough in my own field nothing is dramatic enough for me to deploy this expression. Is "shifting from physics to biology" unusual enough to raise eyebrows? I am not sure without too much context. My gut sense is perhaps not because I'd categorize them both under the school of science and they do go together quite well. I'd think "shifting to biology after spending 10 years studying physics" or "shifting to biology from classical string performance study" may probably deserve a "Why?" Although, this is purely opinion.

The part after the "Because..." deserves attention as well. Make sure it's followed by a very compelling reason, or your "Why?" may be tainted with a taste of self-doubting or still-on-a-soul-searching instead of persuasion.

In any case, expressions like such are unlikely very harmful if used sparingly. They also serve to inject some liveliness into the statement. I'd consider using no more than once in a statement acceptable.