Without looking further, I would say it's because elements bubble up as they're sorted.

Edit: Wikipedia [1] seems to agree:

    The algorithm, which is a comparison sort, is named for the way smaller or larger elements "bubble" to the top of the list.

[1] https://en.wikipedia.org/wiki/Bubble_sort

Because smaller or larger items bubble up to the top of the list. If you watch a visualization you'll see what I mean.