Funders read your bank statements day by day. Each day the account closes below zero counts as a negative day, whether or not a payment bounced. Underwriters use the count as a quick measure of how tight your cash runs.
Many funders set a limit on negative days per month, and the limits differ from funder to funder. A single overdrawn stretch around payroll or a big supplier bill is easier to explain than a pattern across all 4 months of statements.
If your statements show negative days, explain them up front: a customer who paid late, a seasonal gap, a one-time repair. Moving money in from savings before the balance dips helps more than explaining afterward.