The phrase "unexpected interruption" means something that happens suddenly and disrupts what was already happening, catching you by surprise. Full definition
All kinds of jobs, especially entrepreneurship, present unexpected interruptions or sparks of creativity that simply can't be ignored. (entrepreneur.com)
«She [Tikoian] was overwhelmed by how many unexpected interruptions I had,» said Tincknell. (educationworld.com)
Editors are extremely busy and often get sidetracked while working on projects, so your query letter should make a strong enough impact for the editor to remember it even after several unexpected interruptions. (bizfluent.com)