Some individuals do in fact bruise easily, they may have an ascorbic acid(Vitamin C) deficiency or some coagulopathy. Perhaps they are taking a blood thinner medication like Coumadin, ASA, or Lovenox.
Anytime the tissues are traumatized bruising is possible. Receiving a shot IM or SQ is trauma, albeit very little trauma, nevertheless still trauma. Unfortunately though, a common cause of bruising after receiving an injection is poor technique by the person that administered the shot. Blood slowly leaks into the subcutaneous, dermal and epidermal tissues and causes a bruise.