b658aed6__World_Interactions___UESPWiki__Details
[Encounter Name] Looking to join Stormcloaks [Options] Attacking him results in a bounty in whichever hold you are in. If you follow him, he will eventually reach Windhelm and disappear while going through the front gate. [Requirements] Not Eastmarch or Haafingar [Details] 
An Imperial farmer (named Sylvian) walking to Windhelm to join the Stormcloaks, because "I may be Imperial, but Skyrim's just as much my home as any Nords'" and he believes that "Ulfric has the right of it," and because the Empire "banned the worship of Talos".
Skyrim:World Interactions - UESPWiki
Details