Without more detail, all I can go on is physics (and I'm not the best
at that)

I'd guess you are off-balance on your weighting with respect to the
fulcrum. Is this a pendulum-type or a rocker type cradle? If you rotate
it 180 degrees, does it walk the other way? Is your floor level? Are
you inadvertently pushing it when you rock it (applying horizontal
motion rather than swinging motion)?

Depending on your design, the checking of weight may or may not be as
easy as letting the cradle come to a comlete rest and checking the top
with a level. The weight can be thrown off by the rocker rails
hypothetical ceter point not being centered over (under) the center of
gravity.

Stuff to think about.