Staircase phenomenon (Treppe) is due to:

Changes in muscle strength at the onset of contraction due to repeated stimuli are known as staircase effect (Treppe). Although all the possible causes of the staircase effect are not known,  primarily by increasing calcium ions in the cytosol because of the release of more and more ions from the sarcoplasmic reticulum with each successive muscle action potential and failure of the  sarcoplasm to recapture the ions immediately.