Ours always has requred more cranking than I felt should have been required. I recently read a thread here regarding waiting a couple seconds after turning the key to position 2. I usually wait until the dash lights start turning off (maybe 3 seconds) and it works for me. Can't tell you why though especially in your case where it was cranking immediately apparently and then all of a sudden it wasn't. Techron, different gas nor a new starter (obtained under warranty) had any impact for me.
2006 X3 Silver Grey/Black Lthr/Alum Trim/Sport Prem Pkg/Xenons/Servotronic/Prem Sound/Satellite Radio/Heated Seats/Carbon Fiber Pillars/Blue Tooth/Aux In/Cargo Net/Aluminum Pedals/V1 Hardwired