In this case, one of the most contributing factors could be the electrostatic interactions between the charged domains of the protein (pH dependant) and the surface charges on the plastic/polystyrene/polycarbonate surfaces. Hydrophobic polystyrene surfaces generally have a certain degree of positive charge which can interact with the negatively charged proteins, which could also favour a weakly bound protein layer formation. 

Usually higher pH  (>8.0) favours deprotonation and this increases the negative charge density on protein. As the pH of a solution increases, deprotonation of the acidic and basic groups on proteins occur, so that carboxyl groups are converted to carboxylate anions (R-COOH to R-COO-) and ammonium groups are converted to amino groups (R-NH3+ to R-NH2) and hence the net charge on the protein is dominantly negative which can bind to positively charged regions of the plastic/PS surfaces.