Another link, to the reappearance.
"Due to the corrosive effects of salt and humidity in the islands, HQ-9 missile systems must be removed and sent back to the mainland for maintenance periodically," said Timothy Heath, a senior defense analyst at the Rand Corporation.
Simply maintenance related.
Although, seems odd from a strategic standpoint that you would pull all of the missile systems at the same time for repair...instead of staggering the repairs back to back thus leaving some at any given time.