Looks like they return to Mauville City, and we see Wattson's Electrike evolve here. Also, we meet Wattson's assistant's Pokemon, Ampharos! It seems May and him have a battle (she uses Skitty). Then it's also Ash's Torkoal VS Wattson's Manectric...
'Righty then, now for the text, here's my basic translation, it was pretty easy:
Ash and co are headed back to Petalburg City for the 5th Badge, and as planned, they pass Mauville City. They enjoy the town for a bit, shop and stuff and explore as usual. Later, they let all their Pokemon out to play, and as usual, TR comes in and snatches and picks a fight and stuff, and then Wattson comes and helps them (which is where I believe Electrike evolves). Ash then decides to challenge Wattson to a rematch, in order to test the strength of and train his new Torkoal!
Well, now we know the reason for the rematch! Hehe, I bet Ash loses this time :D, but I don't think Wattson takes the badge back if he does. He beat him the first time, fair and square, and earned the badge. It may have been an easy win, but it was still fair and square. :P