Fluid Cavity - Everything symmetric but it moves in inflation step!

[biofriend]

Hi All,

I am modeling a fluid filled cavity. Simply it is a ball, 2D axisymmetric, filled with water.
CAX8R and Neohookean is used for the wall. Cavity reference node defined on the axis of symmetry (in the middle of the half circle).
Boundary conditions, the edges of the ball on the axis can just move U2 which is axisymmetrically necessary. Other than this, Fluid cavity boundary,
internalsurface = the internal surface of the ball

Dimension is in micron, that's the values are so small.
all as it comes here:

.
.
.
*NSET, NSET=CAVITY
100000
.
.
.
*FLUID CAVITY, NAME=CAVITY, BEHAVIOR=hydraulic,
REF NODE=CAVITY, SURFACE=internalsurface

*FLUID BEHAVIOR, NAME=hydraulic
*FLUID DENSITY
1e-15
*FLUID BULK MODULUS
2.2e-3
.
.
.
*BOUNDARY
s1.CAVITY,8,8, 1e-6
.
.
.
As everything is just symmetric, I would expect the ball, as the pressure is building from zero, just to inflate and not to move in any direction. But it just moves down and I really dont understand why. I first thought that it could be due to variation of mesh in the wall and that the solution is mesh dependent but a uniform mesh size in all the 180 degree half circle eliminated this possibility.

I have attached a figure to this post of the simple geometry for better clarification.

I am going mad! and I am very thankful of you valuable insight in advance,
Sincerely,
Bio

[biofriend]

And I think there is no point in fixing the reference point, since it just doesnt move but the cavity moves relative to it and hence it wont be anymore in the center. In other words, even though that the cavity reference point doesnt move, but the cavity does!

Eagerly waiting for your bright suggestions!