Fullerton Police are investigating why an 18-wheeler suddenly veered across lanes of eastbound Imperial Highway early Sunday and slammed into an occupied apartment building, sending the driver and passenger to a trauma center and three residents to the hospital. The collision happened at about 6:03 a.m. April 30 in the 600 block of Imperial Highway, the FPD said in a news release. The 18-wheeler did not appear to be going over the posted speed limit, according to a preliminary investigation. The male truck driver, 44, and a female passenger, 42, were both transported to local trauma centers for treatment. Three…