A desk rejection, before your paper ever reaches a reviewer, usually comes down to a handful of avoidable issues. Here are the ones editors mention most often.
Scope mismatch
Sending a case report to a journal that only publishes original research, or a niche sub-specialty study to a generalist journal, wastes weeks. Read the journal's aims and scope, and check recent issues, before you submit.
Weak reporting
Missing a flow diagram, an incomplete methods section, or statistics that can't be followed all signal a manuscript that wasn't checked against a reporting guideline before submission.
Missing declarations
Ethics approval numbers, conflicts of interest, funding statements, and data availability statements are checked before peer review even starts. A missing one is an easy reason to return a manuscript unread.
The fix is boring, and that's the point
Run your manuscript against the journal's own submission checklist and the relevant reporting guideline before you submit. Nearly all desk rejections are avoidable with this one step.