The examples I know are of intentional experimentation - an axolotl's genetics suppresses the production of a hormone that causes metamorphosis, so administering this hormone may cause it to metamorphose - or, in rare cases, due to prolonged, very bad environmental conditions. These axolotl morphs usually die not long after metamorphosis.
There are also crossbreeds with tiger salamanders that then go into spontaneous metamorphosis, but due to the (recent) crossbreeding with tigers, one might argue that these are no longer axolotls; that argument gets really sticky, though, when one starts discussing what a "pure" axolotl might be.
-Eva