Purpose: Little is known about what triggers an episode of low back pain (LBP) in those presenting to primary care. Previous studies of risk factors have focused on specific occupational settings and work conditions. No study has asked primary care clinicians to consider what triggers an episode of sudden-onset LBP in patients presenting to them for care. The purpose of this study, therefore, was to describe the short- and long-term factors that primary care clinicians consider important in triggering a sudden episode of acute LBP. Methods: One hundred and thirty-one primary care clinicians who were recruiting patients with LBP to a large observational study were invited to participate. A questionnaire was designed to obtain information about the clinician's characteristics, profession and clinical experience. We also asked clinicians to nominate the five short- and five long-term exposure factors, most likely to trigger a sudden episode of acute LBP, based on their experience. Descriptive statistics and frequency distributions were used to describe clinician's characteristics and the frequencies of the main risk factor categories were reported. Results: Based on the views of 103 primary care clinicians, biomechanical risk factors appear to be the most important short-term triggers (endorsed by 89.3 % of clinicians) and long-term triggers (endorsed by 54.2 % of clinicians) for a sudden episode of acute LBP. Individual risk factors were endorsed by 39 % of clinicians as important long-term triggers, while only 6.4 % of clinicians considered them important short-term triggers. Other risk factors, such as psychological/psychosocial and genetic factors, were not commonly endorsed as risk factors for an episode of LBP by primary care clinicians. Conclusions: This study shows that primary care clinicians believe that biomechanical risk factors are the most important short-term triggers, while biomechanical and individual risk factors are the most important long-term triggers for a sudden onset of LBP. However, other risk factors, such as psychological/psychosocial and genetic, were not commonly endorsed as risk factors for an episode of LBP by primary care clinicians. Results of this study are based on primary care clinicians' views and further investigation is needed to test the validity of these suggested risk factors.