Not hard to see
I don't know exactly how much information UCAS have access to, but they probably know which A levels people are taking, which courses they have applied for, and what offers they received.
Presumably they also knew in advance what grades everyone got.
So they could have known last week exactly how many people failed to secure a place.
A week isn't long. But surely some trends must have been visible for months - more applicants (less people taking a gap year because of the fee increase next year), people applying for more substantial courses which might actually lead to a job (but more difficult to get the grades), etc.
Were all these factors static and the extra traffic came as a bolt out of the blue?