Depending upon the age of the chassis, the engine you use and the size of tire, it's not "Un-Common" . . Big tires, and tire shake are chassis killers. Left side of the top tube from the front motor mount back to the rear end housing mounting plates can (and do) crack. The welds at the uprights and top tube are particularily vulnerable if you're shaking the tires at all.
And tire shake is something a lot of guys running the Super Comp combination (Big engine, lots of torque, trans brake) don't notice because while it's brief, it's still there . . and it's a chassis killer. IF . . you rattle the tires so hard (once inna while it happens) that it bounces your head around in the roll cage, give it a good hard look before the next time out . .
So, if you're showing cracking around the gussets, look at the main elements really hard, and if it's a continuing problem, remove all paint from that section, wipe it down with "Gibb's Oil" and check it often . .