As unplanned Emergency Department (ED) return visits (URVs) are associated with adverse health outcomes in older adults, many EDs have initiated post-discharge interventions to reduce URVs. Unfortunately, most interventions fail to reduce URVs, including telephone follow-up after ED discharge, investigated in a recent trial. To understand why these interventions were not effective, we analyzed patient and ED visit characteristics and reasons for URVs within 30 days for patients aged ≥ 70 years. Data was used from a randomized controlled trial, investigating whether telephone follow-up after ED discharge reduced URVs compared to a satisfaction survey call. Only observational data from control group patients were used. Patient and index ED visit characteristics were compared between patients with and without URVs. Two independent researchers determined the reasons for URVs and categorized them into: patient-related, illness-related, new complaints and other reasons. Associations were examined between the number of URVs per patient and the categories of reasons for URVs. Of the 1659 patients, 222 (13.4%) had at least one URV within 30 days. Male sex, ED visit in the 30 days before the index ED visit, triage category "urgent", longer length of ED stay, urinary tract problems, and dyspnea were associated with URVs. Of the 222 patients with an URV, 31 (14%) returned for patient-related reasons, 95 (43%) for illness-related reasons, 76 (34%) for a new complaint and 20 (9%) for other reasons. URVs of patients who returned ≥ 3 times were mostly illness-related (72%). As the majority of patients had an URV for illness-related reasons or new complaints, these data fuel the discussion as to whether URVs can or should be prevented. For this cohort study, we used data from a randomized controlled trial (RCT). This trial was pre-registered in the Netherlands Trial Register with number NTR6815 on the 7th of November 2017.