The application form is a boilerplate document that generally assumes you are leaving some other company to go there, and allows them to do a little digging on what, if any issues, there might have been. They gather preliminary information on someone who is, previously, unknown to them.
That's not the case here, since you're not actually "leaving" anything for the new position. You're staying with the company, and they don't have to dig about anything because your experiences with the company you are staying with are well-known to them. Their standard process is to have an application filled out, so they make you do it to, but how they use it (if at all), will be very different.
So, in this case, it's a formality. Leave it simple - "internal position change for more desired position." Bam. Done, and you don't burn any bridges or ruffle any feathers. OR - since it says "if applicable" - you could also fill in "N/A (not applicable) - internal transfer." Anything bland, generic and not especially informative will do. They won't care. They will care if you say "my current position stinks, I never should have left the previous one."
In the interview they might ask why you want to go back to a position or area you had before. Just state that you really, really enjoyed your time at that position and miss the work, as much as you value the experiences gained in your current one.