Schoolrunner isn't able to directly show you the reasons that you have detention. It's best to ask your teacher about this. However, it may be helpful to look at your slip to see which demerits you've recently earned.
On your "Slip" page, look in the "Demerits Earned" column to see which behaviors may have contributed to your detention. (This column may have a different name, depending on your school's setup.) Learn more about the "Slip" page here.