Why CVOL increases?

Hello,

I modeled a cylindirical Neo-Hookean rubber tube and to be sure that hyperelasticity part of the model works properly I am running some explicit analysis. With a mass flux of water (tried volume flux too) I inflate the tube and at the next step apply a displacement BC along the axial direction of the tube. In the displacement step, flux is deactivated, and one end of the tube is always has a Encastre BC. My problem is that normally I expect an increase in the pressure (PCAV) however during the tension I get constant pressure and an increase in CVOL and also it increases same as the displacement amplitude (I gave the displacement as a smooth step, increase in the CVOL at the tension step is appears to be smooth step as well).

Could you explain me why this is happening?

Thank you in advance for all your attention.