'Nigel' our resident nuthatch doesn't eat upside down. I am very excited as we've never had nuthatches before, he arrived recently and I hope he stays.
There is no mystery. Nuthatches naturally feed by 'crawling' all over tree trunks so need to be able to go sideways and upside down too. Perhaps yours has learned that feeding upside down enables it to keep an eye on the ground for cats while it feeds.
For the record, I am not soft in the head and I don't name all the birds and animals in the garden, well apart from Norbert the bat and Fred the frog.......