This is probably due to the fact that people wanted to get to the first positions in the mempool or simply have extra money that they are willing to donate to miners. 
Which means they are just ignorant or they just want to donate bitcoin to miners that mine the next block which will contain their transactions. Less than 500 sat/vbyte is still highly enough to get a transaction into the next mined block. This is just my opinion. It is a waste of money on fee if not donating it to miners. I also think it was not because of wallet wrong fee estimation. 
