The reason why the phone will getting so hot to touch

[AC Question]

My phone is LG G3. Here are some information about this phone.

Stock setting:

CPU : S801, Qual cores, 2.5GHz
RAM : 3G
eMMC : 64G
Broadband channel : LTE (4G)

Down below is the actual settings of my phone :

CPU : Under-voltage, Reduced frequency ( Below 1.7GHz ) with a battery friendly governor.
OS : Android 6.0.1 (CM 13)

The problem is the phone will eventually getting hot without any sign,
the peak of the CPU temperature is almost 80 Celsius with my setting that I have showed above!

That just freaking me off, and this issue has been stalking me a long time from the day I brought the phone, and now the problem is seem to be more serious these days. Even I set to dual-core and running at 1.4GHz only, the thing still go like that fury. I never met a phone have a lion heart like that. I am going to replace this phone with galaxy S6. But I still want to know why is that happening?

I have used a few phones that equipped s800 series CPU which are never been like that hot, such as Sony Xperia Z Ultra, LG G4, Pentech vega A900.

The reason I still keep this phone in my pocket is because of its changeable battery feature,

 

[belodion]

If I recall right from the early days of the G3 forum, bad SD cards could sometimes cause overheating.