My 10 month old dtr appears to be unable to actively extend her 2nd digit at the DIP jt. I am able to passively extend it fully. There is no signs of trauma or injury. What could this be?

See pediatrician. Was there past trauma, bruising, swelling of the digit? An avulsion fracture with separation of the extensor digitorum from its insertion on the distal phalanx can happen from a blow to the tip of the finger such as from catching a ball. Please take your baby to the pediatrician for an evaluation and referral ASAP. I am assuming this is a new problem.
Could be injury. If this is a new finding, your child may have a tendon or subtle bone injury at the DIP. I suggest you have her seen and she may require Xrays to exclude a small fracture or dislocation.