Disclosure rules require prompt release of material information, so a company that knows it will badly miss cannot wait for the calendar. Pre-announcements are filed on form-8-k, often before the open or after the close.
Negative pre-announcements outnumber positive ones and move prices more, because they arrive without the context of a full report or an earnings-call to soften them.
Example: Northwind Tools pre-announces after the close, cutting quarterly revenue expectations to about $196M from the $206M guided, with the full release still three weeks away. Shares fall 13% in extended trading.
Related: guidance-cut, form-8-k, earnings-report, earnings-miss