A bumblebee is a pairing between a spider and a pastel. So..
A spider produces half of the babies as spider(50%)
A pastel produces half of the babies as pastel(50%)
A bumbleebee results when one baby gets the jackpot of both a gene from the pastel, and the spider. It's all in the odds.

A new morph is a mutation of a existing snake.. just a quirk of the genes at birth(hatching or fertilization if you will). So a new morph could potentially hatch out of any egg at any time. Yes, it's tremendously long odds that any of us will have some new morph poop out of otherwise normal eggs, but that chance does exist(however remote). Then you have to hope it proves genetic, and passes on to offspring so you can replicate it.
Hope this helps.