Ambush

​What factors affect an ambush/being ambushed?
 
In order:
 
Being in heavy terrain (i.e. ambushes are unlikely in the open)
Scout skills (the more the merrier)
Enemy ambushing skills
Size of party (a big party is not going to get many ambushes)
Having help with you (for example some militia, helping you on  a mission)
Skills such as Jungle fighter, if fighting in the appropriate terrain