Dealing or receiving damage from any source (including champions, monsters, towers, and minions) counts as combat and will result in the base movement speed bonus of 25.
There is no range limitation on this damage.
The application of any non-damaging combat debuff (such as exhaust) to an opponent will trigger this state as well.
If invulnerable (such as being under the effects of KayleSquare.png Kayle's Intervention.png Intervention), then autoattacks and non-cc abilities will not trigger this state.
Heals and buffs to allies are not counted as combat.
Damage-over-time spells and abilities reset the timer with each tick of damage
http://leagueoflegends.wikia.com/wiki/Boots_of_Mobility