This folder is created by ArcGIS for Server when an existing cached map service is overwritten. It is a backup of the original cached service with the following naming format: <service>_<timestamp>
ArcGIS for Server creates this cache folder by default.
This option can be changed by modifying the Caching options in the Service Editor to either update the cache manually or automatically. This is described in Step 5 of the following documentation: ArcGIS for Server: Overwriting a service in ArcGIS for Desktop.
Because the original cached map service is generated as a backup, the available space on the drive where the cache is located can grow exponentially. This can lead to performance issues if there is insufficient space on the drive and especially if the ‘argiscache’ folder is located on the system drive (C:). Take this into consideration when selecting the appropriate option to update the cache.