Ladies walk like men.

[sharonasbar12]

What causes say for example Lydia to walk like a man whenever my character is female?

[Kalilies]

Her animations are set to male in the ck. A bunch of female NPCs have the male animations assigned, that's completely normal.

[Killikrates~2014]

There is a mod on the Nexus called Correct Gender Animations that attempts to "correct" that and switch it back to female animations. You can also create or alter an existing mod in the CK and check / uncheck the "use opposite gender animation" box if you are comfortable working in the CK (I'm not much help in that regard, but there are tutorials & such out there).

[Sailing Rebel]

Feminine Females will alter female NPCs so they use the correct gender animations. But read the desc as there are some caveats. NPCs can still occasionally switch back to male animations at random.