Usually, a rubber extension will increase the backpressure a bit. With some engine/muffler combinations, that little bit of added backpressure is enough to make the engine overheat and quit. That my have been your difficulty...especially since it happened when the engine was nose-up...a condition that automatically causes reduced fuel pressure to the engine. Added backpressure, slight leaning, and the engine can overheat quickly. The slight additional backpressure would add to fuel pressure, but not enough to cause the engine to richen enough to prevent the quitting from lean. Nose down, the fuel would tend to "rush" to the engine, so you're getting a richening effect.

Glad to see you found your difficulty!