Possible Causes & Fixes:
Broad Match Keywords Trigger Unrelated Queries
Fix: Use phrase or exact match keywords to improve targeting.
Irrelevant Keywords in Ad Group
Fix: Keep tight, theme-based ad groups with highly relevant keywords.
Lack of Negative Keywords
Fix: Add negative keywords to block unrelated searches.
Overuse of DKI
Fix: Manually test keyword relevance before using DKI.