The relay reversing ESCs typically have one (BIG) transistor for switching the motor on and off, plus a less hefty one for switching the relay, which should only operate for reverse.
When you have a reversing ESC with no relay, there are basically two sets of output transistors, one set for forward, one set for reverse. Each set has a transistor hanging off the ground line (big thick black wire), and another off the positive supply (big thick red wire). This arrangement is often called a "H" bridge, because that's what it looks like on the diagram, top left and bottom right switch on together for forward, bottom left and top right for reverse, with the motor forming the bar across the middle. To get a higher forward current handling capability, it is common practice to parallel extra transistors. If the manufacturer decides that one of the positive transistors can handle the full load, extra forward current might cause an extra negative side one to be added to beef up forward current handling. This assumes that the wire and tracks can handle it. The wire needs a big enough cross section to handle current without overheating, the tracks need a combination of cross sectional area, and a big enough surface area so as to be able to dissipate any heat generated.
If one transistor fails to switch off, when its opposite number switches on, there is a dead short across the power supply. Irrespective of whether the transistors can handle the rated current, Oriental Amps or real world ones, something will give. It may well be that the dead one didn't have a full, proper contact with the heatsink, causing it to cook itself, letting out the magic smoke thus giving the strong wrong pong from Hong Kong. Just thinking aloud.