Honestly I'm not quite sure why it happens either, for me it has only happened 1 month and next month their attributes were increasing again. When it happened to me in the past and I warn player for poor training he got unhappy. I think I've read that growth is in spurts and is not continuous so maybe hes at the end of his current growth spurt. After thinking about it I dont entirely look at it as a bad thing his attribute doesnt drop a point, just equalizes to the current level. Alot of unneeded attributes drop too freeing up PA to be used on increasing the ones we want. Example my ST has attribute decrease across the board including tackling, positioning, marking. Now I can use the freed up PA to increase the more important attributes now.