why the level of HDL decreases and the level of LDL inceases in patients with high cholesterol level ?

Doesnt always..however if you eat a high fat, high carb diet, the fats are mostly converted to low density lipids and unused carbs are converted to fat…this is because of the chemistry involved in reduction, oxidation, and digestion. The high density lipids wont break down as much because of the chemical reaction involved.
The HDL will actually make your arterial walls too "slippery" for the fats and cholesterol is bind. Thats why Fish oil, Flaxseed oil, olive oil are better for you that butter, hamburger, and fried foods.
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RN educator

One can't give reasons, just descriptions of what has been found to occur.

HDL does not fluctuate very much. Its more dependent on regular exercise, proper B vitamin levels in the diet, and genetics.

High cholesterol is primarily very high amounts of LDL and possibly triglycerides.

LDL from excess animal fats, saturated fats, and trans fats in the diet. High sugar amounts, from candy, fruits, sodas, dried fruit, and alcohol can boost triglycerides.

HDL helps transport LDL out of and around the system. When there is too much LDL, HDL is overwhelmed. LDL can be then oxidized and get "consumed" by immune system cells trapped at sites of damaged artery lining, making blockages as they get bigger from more LDL.
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A lot of reading on the issue, since my arteries are really clogged, but my heart is strong, I'm not dead yet, and I'm blessed with excellent collaterals.