The most common cause of small intestinal obstruction is
Causes of small bowel obstruction in order of frequency  : (Sabiston)
I. Adhesions (~60%) Commonest

Malignant tumors (20%) Second most common cause. Most commonly, these tumors are metastatic lesions
Hernias (~10%)
Crohn's disease (~5%)
Intussusception
Iatrogenic adhesions
Trauma
Carcinoma