When at home, a wall can support straight sitting very well.
But basically if not caused by a physical injury it is most likely caused by the mind.
we all know very well the reaction
when someone rant and shout at us.
The shoulders tend to go up, the neck tends to get stiff
and the upper part of the body starts to duck.
It is an automated reaction of self defense.
Lets assume we bring this automated reaction in particular into a silent sitting meditation,
it creates a conflict.
the mind tries to bring the body into this accustomed, automated position,
forces the shoulders up, tries to come back into that stiff neck
and brings the upper part of the body into the position of ducking.
This automated activity of the mind is in contradiction
to a relaxing and loose atmosphere of meditation.
What I like to say is, although you feel it in the body,
it is most likely not caused by the body but of the mind.
Remain relaxed, give your ok, welcome it and continue.