What triggers Solr crawls?

Below is an explanation of the conditions that will start an incremental or full crawl. 

Reasons for crawls

• The polling interval has picked up a content change. The polling interval is a value chosen at site registration. This value decides how often Solr will check if a crawl needs to be done. If there have been no changes then no crawl occurs. A typical change that would require an incremental crawl at the interval would be a content addition or deletion.  
• The following will automatically trigger a full crawl and disregard the polling interval.
  • changes to alias settings
  • changes to manual aliases
  • changes to automatic alias rules
  • changes to metadata definitions
  • changes to user properties
  • other folder level changes

If you are not seeing crawls automatically occur check the Ektron Windows Service logs (search for exception) and search the help center for your exception.