It happened spontaneously across the US, especially when the Crandalls left the north east. John Crandell, for example, was "Crandell" in the census and "Crandall" on his marriage record. His suspected father (no proof linking the two) has his name as "Crandle" on his land purchase record in Indiana and the Census in Indiana, but Crandall on his census in New York.
If I had to guess, with your family, it was the transition of New York to Indiana that caused the switch. I know how I pronounce the name probably would make it sound like "Crandle" or "Crandall" to someone outside of the NY area. All you need is a few people in government writing down what they here and your ancestor going "... ah forget it" instead of arguing over the spelling.