This relates to their respective company strategies for the fiscal year ending in March, with Nippon Life stating that they will focus purchases on 30-year JGBs as the current yield of above 1.8% is "good in terms of absolute level". But they do add that now is "not the time to buy aggressively" as the firm's base case is for the BOJ to begin normalising policy between April to September next year.
For some context, JGBs tend to offer the best option for Japanese life insurers to hedge against their insurance contracts (which are usually long-term) as they eliminate FX-related risks. And so, higher yields would be a good bet in that sense - so long as they exceed the liabilities held.
This has been an ongoing trend this year as Japanese life insurers are surely reducing their holdings of foreign bonds, considering the surging hedging costs amid a weaker yen. The impact on this mostly is for foreign-denominated flows I would say, since they hold combined assets of nearly $3 trillion. As such, the flows are significant. As for the domestic impact, it shouldn't be much considering that the BOJ owns more than half of the JGB market itself (h/t Nippon.com):
