If you read on a little, you'll learn that Harry thinks it might have something to do with vampires.
He had a stab at question five (How was the Statute of Secrecy
breached in 1749 and what measures were introduced to prevent a
recurrence?) but had a nagging suspicion that he had missed several
important points; he had a feeling vampires had come into the story
Quite how/why vampires caused a breach isn't known. The HP wiki postulates that it may have been to do with the real world publishing of a treatise on the tracking and detection of vampires; Antoine Augustin Calmet's Dissertation on Vampires (1749)
The breach in the International Statute of Wizarding Secrecy in 1749
involving vampires may be inspired by the real-world work of
Benedictine monk Antoine Augustin Calmet, who is famous for being
among the first to record stories about vampires in Western Europe. In
1746, Calmet suggested that vampires may actually exist, and in 1749,
Calmet published a treatise that describes how to track them down in
their lairs and destroy them. If so, Calmet (or his source) may be a
wizard in the Harry Potter universe, since only wizards and witches
could possibly commit such a breach.