Amit,
What you are seeing is porous silicon formation on your wafer.
Porous silicon is sometimes formed on p-type silicon in isotropic
etchants, without the aid of electrochemical anodization.

There were several papers published about this type of porous silicon
formation in the early 1980's.  It is more likely to occur on p+ silicon
than p- .  Try decreasing your HF concentration... that is probabaly what
happens when you use ascetic acid diluent.

--Rosemary Smith
  Professor
  Dept. Electrical & Comp. Engr.
  University of CA, Davis