One thing to remember is that Gregg's Brigade wasn't the only unit moving to Raymond. Maxey's 3300 men were following Gregg from Port Hudson, and WHT Walker's Brigade and States Rights Gist's Brigade were both slated to move to Raymond as they arrived in Jackson from the east. In fact, had Grant moved less quickly, the force moving to Raymond could have grown to 12,000 men ... which would have made crossing Fourteen Mile Creek very difficult regardless of the direction Grant turned. Of course, Maxey's Brigade was delayed by McPherson's raid on the railroad to New Orleans
Initially, Pemberton seemed confident that he could wait for Grant behind the Big Black River, while another large force waited behind fortifications in Jackson. Bowen, however, pointed out to him that to do so would be to give Grant interior lines between the railroad bridge and the Bush/Birdsong area, and Pemberton did not have the troops to guard all crossings sufficiently. This convinced Pemberton on May 10th (if my hazy memory serves me) that he would have to move out to Edwards' Station with his mobile wing of 23,000 men and meet Grant in the field of battle. To increase his odds, he ordered all troops at Jackson to march to Raymond, which, if successful, would have placed 12,000 men within 10 miles of his force at Edwards. The two forces would be within supporting distance of each other, and whichever direction Grant turned, he would have to expose his vulnerable rear to the other force.
Not a great plan, but one may note that a similar plan had worked just a week earlier at Chancellorsville. Really, I think Pemberton was just reacting with panic to Grant's moves by that point. As it is, Pemberton himself did not even move out from Vicksburg to Bovina until the 12th, if my hazy memory serves me.