Well in this specific case, the reason is that another system forced me to create suspicious transactions and I might as well rant about that as well.
I lent my son my horsebox when he moved house. He lives and works in Bath, which has just introduced a Clean Air zone where they charge £9 per day for access with my 2004 Renault Master. He assured me that he would not be driving into the area, but in the end he did. 11 times. I had 10 of the notices on the same day, and in spite of the notional 14 day payment window to get the reduced penalty, half of them were dated more than a week earlier leaving me only a couple of days to pay. The online payment system does not let you bundle up payments, each has to be done individually, entering the long PCN code, all your card details, and full name and address each time. By the time half a dozen had gone in, Barclaycard's bots not unreasonably smelled a rat. Because I did not want to risk being charged an extra £660, I really did have to make sure the transactions went through.
I thought it was more likely that they would block the payments if I ignored it, rather than phone me back. The last time I responded that a suspicious payment was not mine, they did cancel the card and call me back to explain.