When I close my mouth, my lower front teeth touches the back of upper front teeth. Currently going through orthodontic treatment but what may be reason?

Not unusual. As your teeth are moving into the correct alignment, it is not unusual for only the front teeth to touch. Certain movements for particular corrections may produce these temporary conditions. If you remain concerned, ask your treating dentist for more specific information of your particular situation.
Normal. Your lower anterior teeth should barely touch or have mimimal space between your inside of your upper teeth. This is considered normal for anterior guidance. Too much space is an over jet (lay man's term is over bite). If the contact is too hard, the anterior occlusion should be adjusted.
Supposed to. Given a normal 25% vertical overbite and a 2 mm horizontal over jet, lower foot teeth are supposed to very lightly touch the back of upper front teeth when back teeth are in occlusion. Hopefully you are seeing a qualified orthodontic specialist. Ask him/her where you are in treatment and what still remains to be accomplished.
Normal anatomy. The relations of your front top teeth should touch the front bottom teeth.