I'm curious as to why would my 2 year old complain of back pain?

See Peds Ortho. It is not normal for toddlers/kids to complain of back pain so i would recommend having him/her seen by a pediatric spine specialist to see what could be the issue.
General peds. Typically, when a toddler complains of back pain, the list of possibilities is completely different from adults or even adolescents. A good general pediatrician is the first stop. Acute back pain with a fever may be a kidney infection. Important back pain may cause decreased range of motion or limping. A focused examination to try to localize the back pain will be key to the next steps.