The usual reasons
Revenue decline. Falling deposits are the single most common trigger, and a seasonal fall that has not been explained looks identical to a real one.
Negative days and returned payments. On a revolving facility these are read as an inability to carry a balance.
Permanent utilisation. A line that has sat at its limit for a year is not being used as a line, and providers reduce facilities that behave like unpaid term loans.
A new UCC filing or a second funder appearing in your statements. It signals additional obligations against the same revenue.
A fall in personal credit, since most small-business lines are underwritten partly on it.
What to do about it
Ask for the specific reason in writing. Providers will usually give it, and it tells you what to fix.
If it is seasonal or has an identifiable cause, provide the evidence — twelve months of statements, the prior year for comparison, an explanation of the event. Reductions are sometimes reversed on evidence.
Do not immediately draw the remaining balance in full. It confirms the concern and can trigger a demand.
Arrange an alternative before you need it. A reduction is a warning, and the worst time to look for replacement funding is after the facility has gone.
Preventing it
Cycle the facility. Returning toward zero periodically demonstrates it is working capital rather than a permanent loan.
Keep deposits in the business account and eliminate negative days, particularly in the months before a review.
Tell the provider before they find out. A planned second facility or a known seasonal dip, raised in advance, is treated very differently from the same thing discovered at review.