Why would all passengers be asked not to eat peanuts when there is one allergic person on board? In this review of BA British Airways Club World London City flight there is one thing I don't understand:


  ... it was announced that a passenger onboard had a severe nut allergy, so they asked people not to consume nuts on the flight.


I've never seen this in a restaurant or any other setting. I gather it's usually the responsibility of the person suffering from allergy to avoid what they are allergic to (usually with a legally mandated help, like known allergens listed on the menu).

I understand that having a person have an allergic reaction to nuts (which can get very bad) itself is a risk to the whole flight. What I'm missing is how other people eating nuts may endanger the one person with allergy.

The only thing I can think of is perhaps some loose nuts flying into the allergic person's meal (or, more directly, mouth) in a severe turbulence. Is that it? Is it a normal procedure to ask passengers not to eat nuts when there is an allergic person on board?
The problem with a severe peanut allergy is that you don't have to eat a peanut to be affected.  With a severe enough allergy, inhaling peanut "dust" can trigger a life-threatening emergency.  In a confined space, any number of events could cause an issue: people dumping peanuts into their hands, dusting their hands off, accidentally sneezing while chewing the peanuts, etc.