Similar thing happened to me when I was still burning miles. Selected seats in first row of Y, long-haul B747, emergency exit row. As I recall had selected sears A/B. when I checked few weeks later saw that we had been moved to E/K in same row. I.e three seats apart both middle seats.
Nobody could explain why and useless Executive Club office in Bremen couldn't do anything to fix it, or even change it. I finally complained to BA's manager in GVA who spent some days fixing the problem.
He never would tell me how/why this happened but my suspicion was that a disgruntled employee was intentionally screwing around high rated exec club members. Either that or their computer hates them as much as I do and doing same thing.
Beware BA.