[Operating Pressure and Blown Drain Cap]

Hello, 

I am new to the forums so I apologize if this question is in the wrong forum.

I went out to the pool and discovered that my drain cap had cracked and was leaking water.  The drain cap is about a year old.

First Question:

What would cause something like this? Any ideas?

Second Question

My normal Operating pressure is 15 when I am not running the water to anything in my system.  However, when I divert water to either my heat pump or solar heater the PSI kicks to 20. IS this normal or something I should be worried about.

I have a hayward S-210T filter and don't know the pump specs, Ill have to check when I get home.  But I figured I would post the question and see what I would get.  Ill update full specs when I get home.  

16x32 size with approximately 20,000 gallons.

 

Thanks.