A 8 years old girl c/o pain abdomen, an investigation showed leucocytosis and u/s acute appendicitis, undergone surgery for removal of appendix and biopsy confirmed acute inflammation. After 3 weeks of surgery patient again c/o pain abdomen u/s showed typhlitis and was advised iv fluid therapy with ceftrioxone and metronidazole iv course.

What could be cause of typhlitis?