It’s because you are have an incompressible flow case. Don’t worry about negative pressure in this case, it’s normal to occur this kind of thing, look, when you are running an incompressible simulation and have tied one boundary to a low pressure or set the operating pressure to 0 Pa (in this case) to cut down on rounding errors, then it is entirely possible that you will have regions of negative pressure in your domain. This is because the pressure gradient enters the Navier-Stokes equations and so it is pressure differences that drive the flow. So, in regions of separated flow, the low pressure inside that region will be relative to the lowest fixed pressure in your system and may well go negative. It works subtracting the pressure in the equation, and so grossly this is the reason for the negative pressure to happen, does make sense?
Of course, in general, absolute pressure can’t be negative, but for an incompressible case (constant rho), you’re dealing with gauge pressures and not absolute ones. If, however, you were to model an ideal gas, then your operating pressure might need to be fixed at, for example, atmospheric because now a negative absolute pressure would result in negative densities and so would make things difficult to say the least.
Please, let me know if it helped you