The adaptors plug should have been properly rated for 13amps. Or an internal connection failed and arcing started. All the same the plug top should have been able to resist the max current draw of the fuse.
Second look, I guess it could have been arcing on the line connection, maybe copper hardening due to heating up and cooling down.
You were but it probably wasn't going to cause a full blown fire. I'm assuming the moulded plugs have some extra qualities and the breaker in the cu should have popped after the current draw got serious.
The adaptors plug should have been properly rated for 13amps. Or an internal connection failed and arcing started. All the same the plug top should have been able to resist the max current draw of the fuse.
Second look, I guess it could have been arcing on the line connection, maybe copper hardening due to heating up and cooling down.