Why Does the Unit Cell Change During My SIESTA Calculation

I am performing calculations using SIESTA, and I noticed that the unit cell dimensions are changing during the simulation. Could someone please help me understand why this is happening and how to control it?

Here are some details about my setup

SystemLabel      1BN-1NumberOfAtoms    8NumberOfSpecies  3%block ChemicalSpeciesLabel    1    6  C.gga    2    5  B.gga    3    7  N.gga%endblock ChemicalSpeciesLabel%block LatticeVectors        5.0205340385         0.0000000000         0.0000000000       -2.5102670193         4.3479100179         0.0000000000        0.0000000000         0.0000000000        15.0000000000%endblock LatticeVectorsAtomicCoordinatesFormat NotScaledCartesianAng%block AtomicCoordinatesAndAtomicSpecies     0.000000000     1.449303000     7.500000000    1    -1.255134000     3.623258000     7.500000000    1     2.510267000     1.449303000     7.500000000    2     1.255134000     3.623258000     7.500000000    1     1.255134000     0.724652000     7.500000000    1     0.000000000     2.898607000     7.500000000    3     3.765401000     0.724652000     7.500000000    1     2.510267000     2.898607000     7.500000000    3%endblock AtomicCoordinatesAndAtomicSpecies# K-points#kgrid_cutoff        0  Ang#%block kgrid_Monkhorst_Pack#1   0   0   0.0#0   1   0   0.0#0   0   1   0.0#%endblock kgrid_Monkhorst_Pack#%block GeometryConstraints#position from  1 to  180#%endblock GeometryConstraintsPAO.BasisSize     DZPPAO.EnergyShift   0.03 eVMD.TypeOfRun      CGMD.VariableCell    falseMaxSCFIterations  400MD.NumCGsteps     0MD.MaxForceTol    0.005  eV/AngMeshCutoff        150 RyDM.MixingWeight   0.02DM.NumberPulay   3WriteCoorXmol   .true.WriteMullikenPop    1XC.functional       GGAXC.authors          PBESolutionMethod  diagonElectronicTemperature  25.86 meVSaveRho        .true.#UseSaveData     true#DM.UseSaveDM    true#MD.UseSaveXV    true#MD.UseSaveCG    true

The cell after siesta calclution :

         13.370161376       0.000000000       0.000000000          0.000000000      11.036039476       0.000000000          0.000000000       0.000000000       9.184564484           8  1     6       0.000000000       0.131324557       0.816587440  1     6      -0.093875755       0.328311439       0.816587440  2     5       0.187751436       0.131324557       0.816587440  1     6       0.093875755       0.328311439       0.816587440  1     6       0.093875755       0.065662324       0.816587440  3     7       0.000000000       0.262649205       0.816587440  1     6       0.281627192       0.065662324       0.816587440  3     7       0.187751436       0.262649205       0.816587440

Observation: The cell change completly