I faced the exact same issue last week and it was due to bad traffic from specific IP. There were lots of interactions from a single contact and in one shard.
Marketing Automation service was unable to process all the interactions and service started throwing SQL timeout error and due to that DTU utilization for one shard was 100% for 2-3 days.
For further analysis, I started reviewing SQL tables for xDB, then found that that AutomationPool table in Marketing Automation is holding lots of records for a bad contact and that was causing 100% DTU utilization.
Then I performed below steps to fix the issue-
- Stopped Marketing Automation service (This brought DTU utilization down to normal state)
- Ran query to find out which contact has more records to process
SELECT [ContactId], Count(contactid) Total
GROUP BY ContactId
ORDER BY 2 DESC
- Found a contact id which had around 70+ entries
- Deleted all records for that contact from AutomationPool table
WHERE ContactId = 'XXXXX'
- Restarted MA service
Hope this helps.