Complex sentence: Niles, initially, is quick to back out of the responsibility and leave Frasier to shoulder the burden (and Martin's anger) all by himself... until he learns that Martin intends to bring Sherry as his date to Niles' awards ceremony.
Split into simpler sentences: Niles, initially, is quick to back out of the responsibility and leave Frasier to shoulder the burden (and Martin's anger) all by himself. However, he soon learns that Martin intends to bring Sherry as his date to Niles' awards ceremony.