I can think of a few reasons:
1) He sees value in BNSF;
2) He want to use some cash so that his successor won’t have too much cash to squander (this is legit considering his age, and a lot of times people make mistake when they are rich);
3) Last but most important, financial crisis taught Buffett a lesson: liquidity is very important. GE and GS got his endorsement and got cash by issuing stocks in one day. Buffett could not do that before buying BNSF and the stock split. Now he can. BRK is S&P component, much more widely held. “Get access to capital market” is one reason Goldman went public in 1990s.
I think Buffett only said 1. But I think 2 and 3 are also the factor here. Remember Buffett was against stock split up to this BNSF deal. Although he said publicly splitting Berkshire stock is for small BNSF shareholders, a valid point. Nonetheless I would be astonished that was the only reason to overthrow 40 years belief/rule in one deal.