Lexical bundles, recurrent multiword combinations in a register, are extremely common and important discourse building blocks in academic writing. An increasing number of studies have investigated lexical bundles in academic writing in recent years, but few studies have explored L2 learners’ interpretations of their own bundle production, particularly sentence initial bundle production. Investigating the sources that have appeared to influence learners’ choices and knowledge of bundles is important as it complements what we know about the structural and functional features of lexical bundles and provides useful first-hand information for second language writing pedagogy. The present study interviewed five Chinese postgraduate students to probe possible reasons for their use of the typical sentence initial bundles identified in the self-built Chinese Masters and PhD thesis corpora. The interviews revealed diverse explanations including interlingual transfer, classroom learning, noticing in reading, a lack of rhetorical confidence, and misunderstanding of rhetorical conventions. The results suggest the need for raising students’ awareness of the common sentence starters in postgraduate academic writing, increasing their confidence as student writers, familiarizing them with rhetorical conventions, and incorporating effective corpus-based tools into pedagogical practices.