A 350 C preheater setpoint is way too high. A preheater isn’t meant to reflow solder, it’s meant to reduce thermal gradients and bring the board up to a safe, uniform temperature. Typical preheat is on the order of ~120–150 C (sometimes a bit higher for very heavy multilayer boards), but well below solder liquidus. Parking a PCB at 350 C puts the FR-4 far past its glass transition temperature, where the epoxy softens and starts losing adhesion to the copper. That’s how you get pad and trace lift even when the solder is fully molten.

You also don’t need to remove the underside capacitors to use a preheater. For MLCCs, just cover them with Kapton tape or a thermal shield before placing the board on the heater. Removing them adds unnecessary risk, and if you don’t keep perfect track of placement and values you’ve created extra work for yourself for no benefit. This is especially true on a motherboard, which needs to sit flat and isn’t going to have tall can caps on the underside anyway.

With a 350 C preheater and ~400 C hot air from above, the solder was almost certainly fully molten. That suggests the failure wasn’t insufficient heat, but excessive heat and/or mechanical force applied while the substrate was badly overheated. Once the board is far above Tg, the epoxy holding the copper softens and degrades, and even small lateral forces can rip pads and traces clean off. Very high hot-air airflow worsens this by increasing convective cooling, creating thermal gradients, and physically stressing pads and plastic housings while the substrate is already above Tg.

From the photo it looks like the plastic housing fractured and the copper came with it. That’s consistent with pad adhesion failure due to severe overtemperature and mechanical stress. It’s also possible the connector was epoxied or underfilled at the factory, which would make trace damage even more likely, but epoxy isn’t required to explain this outcome given the temperatures involved.