Could be based on an update to their risk model. Apple and TSMC are relatively closely tied together so having both TSMC and Apple increases risk concentration

Could be based on an update to their risk model. Apple and TSMC are relatively closely tied together so having both TSMC and Apple increases risk concentration for example if TSMC butchers a node release Apple will suffer for at least a year or two. Berkshire probably decided they were no longer comfortable with the risk concentration of having both.