What confuses Bilbo for always about the day he rushed off to join the dwarves?

From the text:

To the end of his days Bilbo could never remember how he found himself outside, without a hat, walking-stick or any money, or anything that he usually took when he went out; leaving his second breakfast half-finished and quite unwashed-up, pushing his keys into Gandalf’s hands, and running as fast as his furry feet could carry him down the lane, past the great Mill, across The Water, and then on for a whole mile or more.