Because Soveishit has a "brilliant" plan. He will divorce Navier for a year, will make Trashta his empress, so his son will be born in wedlock and is his legal heir, then after a year, he will dump Trashcan, and remarry Navier. So he will get his legal heir and keep Navier. Except he forgot to explain his plan to Navier...
the kicker to this """""brilliant"""""" is that he knew that trashta wont be a good empress so he expects the citizens to want him to get back together with navier so he would use that as an excuse to remarry navier and make her an empress again. guy just using women to his own gain and expects navier to stay with him, navier is too smart for him.
》This may be contain spoiler《
Sovi will later tell Navi why he had divorce, it is written in a letter and navi is in the western kingdom that time already. It's on 2 to 4 or 6 more chapters if i remember it correcly (づ￣ ³￣)づ
He missed out a big part of this plan. This explains bout the brief divorce. Did he ever think bout how Navier is super hot to wait for him? Well, it's okay as Navier get new husbandd uwu ε=ε=(ノ≧∇≦)ノ
Im seriously confused with him. Please use ur head. Damn stupid king.