The Bank of Japan is buying JGBs in order to drive yields lower. Yields have been rising, not just in Japan but across DMs.
Part of BOJ policy is yield curve control, the Bank is targeting those in the 10 year but in doing so is seeing higher yields leak out into other maturities. Hence, the Bank buys, in previously announced actions, across other maturities to try to dampen the yields.
The key point of all this, for those trading yen, is that while Japanese authorities have been talking up the yen, and even spending huge sums (3.6tln yen is the number from last week's intervention) to try to prop the yen up its widely recognised that the currency will remain weak while BOJ policy remains ultra-loose. Unscheduled bond buys are a sign of this ultra-loose policy continuing, and thus the pressure for a weaker yen will remain.
