Purely from memory (so not a hard fact and I might stand corrected) this is what I often see since several weeks. Why?

Speculation:

The last couple of weeks have seen unusually frequent sloshing back and forth between speculative and defensive moods. Berkshire is seen as quintessentially defensive, so the few people who trade stocks will buy it when the mood swings that way. More often it is money going into an ETF representing the mood of the moment, most of which are cap weighted, so Berkshire gets pulled along with whatever index or sub-index or sector the fund tracks.

The more surprising thing is the longer term: Berkshire's market price return has been essentially market tracking over quite long periods, but in particular almost all of the last 2.8 years. No sloshing there. As above, it seems nobody buys individual stocks any more : )

Jim