Definitely agree. I believe that defending their holdings is absolutely necessary. Even if they don't care whether you will lose your titles, they care whether their castle will be looted, people killed, cattle stoled, property damaged, buildings put out of commission etc. Plus prestige and pride considerations. This should be top priority, #1, the way I see it.
#2 would be participation in realm defence when the CB doesn't involve the liege as a person (replacing with a claimant, excommunicated ruler etc.) but something along the lines of invasion. Especially by somebody from a foreign religious group or even foreign culture group or even foreign culture (but vassals should be less likely to help a foreign liege especially vs an attack from their own religion and/or culture, that being different from the liege's). Also participation in defence against CBs directed against that particular vassal (claims etc.), or war as an attacker when the liege is pressing that vassal's de iure claims (de iure, not pressing your claim in order to depose your well-beloved elder brother and bring his lands into your liege's vassalage). Fellow dynasty member defence against a claimant from outside the dynasty should probably belong here or even higher. Your heir should be interested in keeping you in power, preventing succession law change via faction system, preventing territory loss, supporting territory acquisition. Husband or lover of a female ruler should generally defend her. I can't imagine husband who is her lover not rushing to defend her unless Craven or something.
#3 would be cases like loyal vassals (high opinion) defending their liege against claimants, kinsmen of liege backing him up in most circumstances, vassals not wanting a rival claimant to succeed (e.g. cruel excommunicated kinslayer) vs their current liege (slothful and gluttonous as he may be). I think strong claimants should be inclined to join your defence against weak claimants when they don't want to press their own claims.
#4 peace-loving vassals might appreciate medium authority and be reluctant to allow a rebellion against it. Bishops could help out with crusades, holy wars, excommunication attacks etc.
#5 your traits, especially Just or Arbitrary, should have some bearing, other than respective opinions. To lower the authority of an Arbitrary king could be tempting even to paragons of virtue. On the other hand, many people would be dissuaded from trying to lower the authority of a Just king without having a selfish reason (or some other plausible reason).
#6 if CB allows land to be taken, the potentially outgoing vassals should take a look at the attacker/claimant's laws and compare taxes and levy sizes.
#7 Noble Customs, Religious Customs, Trade Practices should play some role, possibly tech level (to reflect reluctance to be ruled by a perceived barbarian), especially for Byzzies and Italians.
