Before the planned strike in 2007 (which was called off) it was reported that cabin crew had called in sick in their hundreds even before the planned day of the strike.
I would not be surprised if many had called in sick (but the quoted numbers seem high to me). Partly for historical reasons and also because it is an easy way out. Though, I don't feel particularly sympathetic towards any members that had voted for industrial action themselves. If they weren't prepared to walk out they should have voted in the first place.
I suspect there may be a high volume of cases because BA procedure requires the formal lodging of a grievance.