A "reasonable explanation" refers to a logical and rational answer or justification for something. It means providing a sensible reason that makes sense and is easy to understand. Full definition
I'm saying that's the most reasonable explanation for why his numbers stay the same no matter how much the other variables around him change. (sbnation.com)
He gives reasonable explanations of how computerized trading works, and what factors they look for in designing trading systems. (alephblog.com)
At the hearing, the plaintiffs were unable to provide reasonable explanations for the excessive delay in proceeding with the action. (zeilikmanlaw.com)