B-cell and T-cell activation by parasites
I've found a great article that says:


  Mitogens   and   superantigens   have   been   described   to
  explain the strategy used by microorganisms to avoid the
  host specifc immune responses and to ensure persistence.
  These  moieties  are  responsible  for  the  initiation  of  non-
  specifc  (polyclonal)  immune  responses. 


Mitogens are chemicals (usually proteins) that promote mitosis. Superantigens (sAg) form a subclass of antigens and are capable of inducing massive polyclonal T-cell activation and cytokine release (Cytokines crucial role in immune system mediation). Their ability to inflict massive T-cell activation comes from the fact that they bind directly to MHC-II without any pre-processing, and stimulate T-cell activation.
Source: https://escholarship.org/uc/item/47g8w51m
Also there are B-cell sAgs that stimulate B-cells, and their activation.

These altogether (mitogens cytokines, sAgs) result in rapid expansion of B-cells and T-cells.

Also this paper might help to understand why parasites trigger polyclonal T and B-cell activation. Namely to avoid host specific immune system, dilute the pathogen specific anti-bodies. It is important to note that chronic infections could lead to several auto-immune diseases.