It was a special military operation. They were assisting pro-Russian milita's. I have been saying this from the start. Yes, most Russian forces are currently in southern Ukraine, which is why Ukraine was able to easily cruise through Northern Ukraine.
Because Russia is fighting a proxy war with the West and there is no way to know how this will end. They are covering their bases. Those being called up to go to Ukraine are reserves, not conscripts.
Like I said above, conscripts will be trained in the coming months in case this blossoms into something much bigger. They aren't being sent to Ukraine, atleast not as a fighting force, maybe as peacekeepers in Russian held areas. Russia has plenty of trained troops and reserves to call on.