Long unbeaten runs against a particular opponent end suddenly and completely rather than tapering off. The pattern is consistent, and the reason is that the run was measuring something that had already changed.
The run reflects a quality gap, not a hex
Most extended sequences between two clubs coincide with a period when one was clearly stronger, often because it was competing for titles while the other was mid-table.
The results were therefore the ordinary consequence of a difference in squad quality, repeated across several seasons and later described as a psychological pattern.
When the gap closes, the run has nothing left holding it up. It ends at whatever point the two sides next meet.
Sample size creates the illusion of meaning
Two clubs meet twice a season, so a run of ten matches spans five years. Almost everything about both teams changes across that period.
A sequence of that length is easy to generate by chance between mismatched sides and impossible to generate deliberately.
The attention it attracts is a function of how rarely sequences are noticed while they are short, which is a reporting artefact rather than a football one.
Why the ending looks abrupt
Football results are discrete, so a run does not decline gradually the way a batting average might. It is intact until the moment it is not.
The underlying trend, meanwhile, has usually been moving for a while, visible in league positions and in the pattern of the recent meetings.
Narrow wins late in a long run are the warning sign. By the time the streak breaks, the reason has often been present for a season or more.
The psychological account is mostly backwards
Players are said to carry the burden of a record, and squads turn over enough that most of a team has no personal history with the fixture at all.
Where pressure exists, it tends to sit with supporters and coaching staff, who are the parties with actual continuity.
Any effect on the pitch is more plausibly about a coach setting up cautiously because of the record than about players being intimidated by it.
What to check when a run is quoted
Compare the league positions of the two clubs across the run and now. If the gap has narrowed, the run has already lost its cause.
Look at the margins in the most recent meetings, since a sequence of one-goal wins is a different object from a sequence of comfortable ones.
A run held up by a gap that no longer exists is not a reason to expect it to continue. It is a reason to expect it to end soon.