One theory: Career risk.

Most short term relative price movement in large caps is driven by big portfolio managers, specifically the subset who are active.
They are almost all evaluated on a calendar year basis.

The theme in 2023, especially the latter part of the year, was the Magnificent Seven. If you weren't in those, you were underperforming horribly and soon to be fired.
Throughout the last quarter, the portfolio managers were steadily moving more and more money into them, necessitating selling out of other large positions, probably larger caps.
Three reasons for them to pile in: to chase the trend, to hug the index more closely late in the year to avoid even more career risk, and window dressing to avoid holding any losers at year end.

In the new year, portfolio managers like to start off taking more risk, moving into what they think will actually perform well in the year. Leave hugging the index for late in the year.
In this case it seems they think Berkshire fits the bill...at least for a while, as a relative-to-market rebound play.

Thus endeth the theory : )

It fits the observation that the relative-to-market performance of Berkshire's stock seems to change direction right around year end far more often than chance would suggest.
If it was beating, it starts to lag. And vice versa.

Jim