A full crawl kicks off after performing an action that should trigger a incremental crawl
Microsoft Search Server
This issue has only been reproducible when the crawl interval is set to a low value (5 and 15 seconds). If users are publishing content at a constant rate, eventually a full crawl will kick off. This appears to be a Search Server issue, where rapid calls to the "StartIncremental" API eventually result in a full crawl. Set the crawl interval to 300 or more seconds. This behavior was not reproducible using this interval.
In the case of Solr this is usually not a malfunction but due the conditions for a full crawl being met. If any of the following data is synced or modied a full crawl will be requested instead of an incremental craw.