If the footies are pulled tight against the bottom of baby's feet, baby's step reflex can be activated and she will begin walking in the carrier. The step reflex is present from birth to about six weeks and then begins as a voluntary behavior around eight months to one year old. Too tight footies can also make baby uncomfortable and squirmy.
Some fixes are:
1. pull down the legs of baby's footies after securing her in the baby carrier ~ this is a good thing to check with any device that you "secure" baby into
2. make sure baby hasn't outgrown her footies ~ if baby's feet, when extended out, pull the pajamas taut from neck to toes, baby is ready for the next size up
3. use baby legs or knee high socks to close the gap between toes and non-footed pants ~ baby legs can also be pulled down over foot/toes to form "socks"