What makes heat change over to aux heat?

depending on how your installer wired it
when you turn the unit on, it could use both to get to temp as quickly as possible
when you are out of balance, the outside temp is lower than 20 degrees, not alowing any heat to be captured by the heat pump
or when your compressor goes inot lockout, but thats a rare wire situation
finally again depending on wire install and t sat as a user option