"The last 2 months have seen particular jumps, but that may in part be explained by the dramatic increases in hash rates again. "
How is hashrate related to that? It's the speed of block transactions that matter, no?
When the hash rate is increasing between difficulty retargets we see blocks being solved more quickly. Instead of, say, 10 minutes on average we see blocks taking 9 minutes, so this means there's more capacity available to handle transactions. This reduces the pressure on transactions and therefore means that it's much more likely that low or zero fee transactions will get included in blocks by block makers (because they don't have anything better to do with the block space).