Although most Greenland glaciers have undergone thinning and frontal retreat in recent decades, the tidewater glacier Eqalorutsit Kangilliit Sermiat in South Greenland departs from this regional pattern. This outlet glacier has advanced its terminus by almost 2 km since the mid-20th century and thickened persistently with ever-increasing rates, in contrast to neighbouring glaciers. Actively pushed lateral moraines with vegetation and absent trimlines provide visible evidence in the landscape. This study investigates potential explanations of the glacier’s anomalous behaviour, emphasizing surface mass balance (SMB) and reduced frontal ablation. A negative SMB trend and the relatively short modelled frontal response time to abrupt climatic changes rule out SMB as an explanation for the glacier’s behaviour. Oceanic forcing is also considered unlikely due to warm Atlantic water reaching the terminus. Sedimentary processes such as morainal bank shoving or sediment erosion and redeposition potentially reduce frontal ablation or provide buttressing, but evidence is lacking due to limited bathymetric data near the terminus. However, the observed thickening, slowdown and advance is consistent with the modelled response to an advancing terminus on a retrograde bed slope, highlighting the complex interaction between bed shape, calving dynamics and ice flow as key controlling factors.