Stuns have a chance to occur on any hit, but some things make them more likely: (some of this is speculation)
-Critical hits
-attacking a weaker opponent
-using two-handed weapons
-getting a surprise bonus (usually from a successful ambush)
-Opponent is not dodging or parrying
-certain skills and enchantments increase stun or crit chance
As you go for 400 days or getting all warbands to rank 10, you are sure to get this.