Snapshot Graph
You can construct a filter which lists your cases and shows the Occurrences column. Export that to Excel and create a chart which shows you which issues come up frequently.
I have opened a case to allow using Occurrences as a break-down in Reporting charts.
Occurrences Over Time
Using the method above, you can then add a date range search to your filter to show cases edited during a period, export that, adjust the range to the next period, export etc. Putting the data together in Excel you can look at changes over time.
I realize this is a tedious process if you have more than a few ranges you are interested in. I have opened a case to allow display of Occurrences for a time-based line graph Report. The time-based graphs give you a sum for all cases in the filter, so you would need to filter in such a way that you get the number you need.
Workaround
Because the number of occurrences is not put in a BugEvent (edit) when it is updated, there is no history of it in the database. For now, the history is only available for submissions made before fStopReporting is set.
To report on these now, you can access the database directly or via a plugin to look for BugEvents for cases with entries in the Scout table.
You can also use the XML API as mentioned in other answers here.