Alice was unhappy being a pet yukkuri, and swapped places with a stray Alice at the first opportunity. She had a romanticized view of wild yukkuri life, and wanted to taste freedom. She has dreams to become someone important, was confident that as a pet yukkuri she will have superior status and knowhow compared to the strays.
Unfortunately, reality showed her otherwise. Raised as a pet yukkuri all her life, she was ill-equipped for life outdoors, and was rejected by the herd of wild yukkuri. Worse off, she could no longer go back to the life she had before. Rejected by other yukkuri and unable to live as a wild yukkuri, with the last of her strength she threw herself off the bridge in despair. Finally she did become something of use -- as food for the fishes in the water under the bridge.
To quote the last lines of the anko:
"A bird should not be caged.
But she is only a bun, not a bird."