What are the causes of insomnia during pregnancy?

Change in hormonal levels to blame. Plus, the very nature of sleep changes with the course of pregnancy.

Often a deficiency of vitamins and minerals affects sleep (after all, in pregnant women reserves are depleted faster). Check with your doctor. Eliminate the deficiency of vitamin D3, B12, folic acid, iron, magnesium, calcium, B vitamins.

The answer is hidden in the question. The reason is pregnancy. Or rather, in a change in the harmonic background due to pregnancy.
More reasons may be:
An increase in the volume of the abdomen, it becomes difficult for a woman to find a comfortable position of the body;
Increased urge to urinate;
shortness of breath caused by hormonal changes ;
Rapid work of the heart;
cramps in the calf muscles and lower back pain;
Disorders in the digestive system (constipation, heartburn).
Do not take sleeping pills during pregnancy without consulting a doctor. You can normalize sleep by the following actions:
Walk before bedtime.
Normalized nutrition. At the same time.
Normalization of the regime.
I took most of the information from https://kosnu.ru/beremennost-i-son/

Most often, insomnia begins in the first trimester. This is due to hormonal changes, in particular due to an increase in progesterone levels. The body begins to actively prepare for childbirth, and this prevents it from relaxing.

Sleep normalizes in the second trimester. The expectant mother can have vivid dreams.

In the third trimester, a woman becomes anxious, she is worried about the upcoming birth, and this can cause nightmares and insomnia. It must be remembered that insomnia is not a disease; it will disappear if its causes are eliminated.