Can't keep back straight

I've done a few workouts and I noticed when I was doing Flexibility day 1 that I couldn't keep my back straight while bending. I can't keep it straight whenever I'm supposed to stretch to my toes and I'm wondering why this is.

I'm unsure if it's a matter of technique or muscle weakness or posture but I can't seem to keep it straight and when I stretch further I always end up losing form. It becomes curved.

Does anyone know why I'm having this issue? Any suggestions?