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A fitting at the fuel rail loose, cross-threaded or with a damaged seat, so the joint leaks. It is the same pressure fault as a cracked line but with a different repair, and separating the two matters because one is a torque and sealing-face question and the other is a new part.

Torque is the thing people get wrong. These fittings seal by deforming a cone against a seat, and they have a specified torque that is neither guesswork nor "as tight as it will go" — undertightened leaks, and overtightened deforms the seat so it never seals again, which turns a rail into a replacement.

Look for the leak with the engine off and the system still pressurised, since these systems hold pressure long after shutdown. A fitting that has been disturbed recently is the first suspect, and the seat should be inspected for scoring or a witness ring before a new line is fitted — a perfect line into a damaged seat leaks exactly as the last one did.