Investment banks typically defend their IPOs by buying on dips. They do this to protect the institutional buyers they convinced to buy pre-IPO. The investment bank has no intention of keeping these shares; they will mercilessly flog them to whomever they can. Regardless, they often end up with inventory of underwater shares.
Mark my words because this happens every time an IPO does not fare well. The investment bank will stop buying within three months of issuance. They are only willing to take so much risk in exchange for their fat 7% commission. Those blocks you are seeing are most likely the work of the investment bank. This artificial support will come to an end. Don't be left hanging.