If all the Pressure load multipliers were negative, that means a straight pipe with fixed ends will not buckle with positive pressure. It will only buckle by negative pressure (partial vacuum). Positive pressure can cause the stress in the pipe to exceed allowable values, which is a failure unrelated to buckling.

Put the pressure only in the Static Structural model. You might find that increasing pressure will cause an increase in the Load Multipier on a Buckling analysis that only has a Temperature load.