Delays usually result in something arriving late. Why would it result in a return? We have no brexit here, but plenty mailing delays, especially due to covid rates and different countries' reactions to them. No air ems from Japan, for example (though all of the private delivery companies continue to work in and out of there by air).
DPD was not clear about the reasons but, quoting from their reply:
" I cannot apologise enough about the problems with delivery. I'm afraid we are seeing quite a lot of problems with parcels being exported to Europe at the moment, as a result of the Brexit changes, which is why your parcel has been returned to sender.
I appreciate how frustrating this must be, and I'm so sorry that your parcel is again showing as on its way back to the return address. I believe that the issue that caused this has now been resolved, but if you'd prefer for us to refund your order in light of the problems I understand entirely, and can of course arrange this for you once you've received the parcel back. "
So, I am sending it for the third time, hoping the "issues", whatever they were, are now solved.
Regarding delays only producing delays in delivery, rather than returns, this might well depend on the courier.
DPD-local is the cheapest option to send to the EU, at least as a normal customer booking door to door collection for single parcels.
Other options cost upwards of 60% more. The difference was even more pronouced before January, with Royal Mail charging £65 for the same parcel DPD-local woud deliver for £19 or so.
Delays are not going to be cost free to the courier. Keeping a truck stuck in Dover for an undetermined number of hours or days means paying for the driver time and other associated expenses, while jeoparding the logistics. The delay might be determined by government priorities putting parcels ike mine at the very botto of the queue for custom checks and other procedures. Keeping the package at the depot awaiting also cost money because contingency space runs out quickly, and getting an extra warehouse is a long term investment.
So, some cheap couriers might find it less expensive to simply return the parcel and offer to refund. After all their local warehouses and drivers are not paid extra to do that, or very litte more compared to the alternatives. It should be said that they only did something about it when I chased them aggressively, it is not like I received any explanation or refund offers upon receiving it back.
Which again points to costs saving, the less people asks for a refund or a rebooking imediately, the less manageable it all becomes for the company.