Being the last author as an undergrad
There are at least three basic author order schemes:

1) Alphabetical

2) In decreasing order of contribution

3) Split into junior and senior contributors, with juniors coming first in decreasing order of contribution, and then seniors in increasing order of contribution. [Often with just one senior spot]

In Computer Science, I've predominantly seen 1) and 2). An undergrad being at the end of an author list that is clearly not alphabetically ordered is a dead give-away for order type 2), and hence does not require particularly explanation.