They do drop lower leaves naturally, but if this has happened quite quickly, barring you having moved it or got it cold, I'd say it was too dry.

Morning,

Hello Karla,

My lower D. marginata leaves are yellowing, too.

As SJ said, it's natural for lower leaves to drop, but it's possible there could be other causes.

Since days shortened, soil needs to dry, 'almost' thoroughly. If soil stays wet for prolonged periods, leaves yellow, starting from lower, working up.
However, I mist leaves since Dracaenas need humidity.

Brown leaves are caused by under-watering, lack of humidity, or too much heat.

During winter, I treat my marginata as a succulent. Once days lenghten, soil is watered more.

What type of light are they getting?
In summer, my marg gets bright-indirect sun..
During winter, it's placed closer to a window... a little artificial light at night, too.

Remove yellow/brown leaves. They'll drop on the floor anyway.

Good luck, Toni