The iron of a transformer core keeps part of its magnetisation when the field that created it disappears: that is remanence, and it follows directly from the hysteresis loop of the material. It is left behind after injecting direct current into a winding — winding resistance measurement being the most frequent cause — and also after a disconnection from the network at an unfavourable instant.
It has two effects, both unwelcome. It skews any later test that depends on the magnetic circuit: excitation current shoots up, turns ratio readings become unstable and short-circuit impedance no longer matches the history. And if the transformer is energised with the core magnetised, the inrush current can multiply and trip the protection.
The remedy is demagnetisation: applying a sequence of decreasing amplitude and alternating polarity that traces ever smaller hysteresis loops until the core is left neutral. Modern test sets do it automatically at the end of the measurement, and it is worth checking the function is enabled before signing off a test campaign.