Why do miners go offline? As per my understanding mining process in bitcoin is fully automated. A manual intervention is needed only when:
(i)  A hardware failure occurs
(ii) You might want to cool down the hardware
(iii) You completely shut it off (can be due to various reasons)
Are there any more reasons why a miner might switch off his/her system given that the complete process is automated?
One of the most common reasons for which miners go offline is when their profitability drops below the cost of electricity - this could either be the result of the BTC price dropping, or their local operating costs (costs including air conditioning, power, networking, hardware depreciation, etc.) rising, or the total network hashrate rising faster than they are able to acquire a new set of machines to proportionally increase their hashrate.
Remember that most miners aren't usually operating a single machine - they usually operate a mining farm, where a single machine's hardware failure or thermal shut off won't actually take the miner's entire operation offline, just reduce their effective hashrate. For a noticeable removal of hashpower from the network, such as when an entire farm is shut off, the cause is usually economical.