What external forces impacted Honda during 2011? Were these environmental forces really beyond ...

Competitive Forces:  New and redesigned brands from the competition.  This should not really catch a company like Honda by surprise.
Higher incentives offered by competitors.  This is a marketing challenge which could be easily countered, but it sounds like Honda was slow to measure the impact.
Social Forces: Lack of consumer sensitivity to gas prices.  You could argue that they should have research to figure out the level of sensitivity to gas prices.
All in all, these issues were not extreme bad luck, more a matter of Honda not being as in tune to the marketplace as they could be, or not reacting as quickly as possible.