It's not only the receiver who can put a transfer on hold. Mister Tango did put my outgoing SEPA transfer on hold and requested "documents" before releasing the transfer. My explanation of the purpose of the transfer wasn't enough. They need documents. It was my second large transfer. The first went through without any questions.
Its not just Mistertango. Im sure every bank flags and manually reviews large transactions. Have a look at this interesting list of flagging reasons:
https://www.ffiec.gov/bsa_aml_infobase/documents/red_flags/deposit_acct.pdfMistertango might have an automated transaction surveillance system which checks for red flags. If your SEPA transfer is captured for review, then there must be a reason. It could be the source of those funds. Was it an inward SEPA transfer from another person? They usually ask for documents in those cases. Another is, as I mentioned earlier, the amount. If it exceeds a certain threshold, then it might ring alarm bells.