There are two reasons that quickly come to my mind --it could be a [mistake or urgency].
Speaking of nefarious, I dont think it is possible that someone is intentionally paying high fees to hide some nefarious activity. Just like perhaps they be trying to obscure the source or destination of funds by flooding the network with lots of low-value transactions with high fees --something like that?
No, I used the word nefarious in the sense that someone is intentionally clogging up the mempool to make normal users pay outrageous fees and thus make Bitcoin less attractive.
I don't think it is a DDoS attack --using high-fee, low-value transactions would not be an effective method since the transactions would still be processed by the network, even at a higher fee.
I used the the term DoS (Denial of Service) loosely, in the sense that it becomes prohibitively expensive to transact smaller BTC amounts.