I had a period a couple of years back where I my wife and I bought W tickets and got bumped to J almost every time. In all cases it appeared to be because they had upsold to WT pax at check in for a few hundred quid. Easier to upsell that amount and bump someone into J than to ask for a grand for a WT+ person to go to J.

As to why we were always getting bumped, others on here would no doubt have a better insight than I but I assumed that as my wife was parachuting from gold just as I got mine, we would often be the pair in that cabin with the highest total lifetime TP and / or status. But not sure. It's still a bit of a mystery to me all these years later franky.