Fundamentally: Banks are charging negative interest on deposits (EUR / CHF come to mind). With expectations that ECB deposit rates may be cut further, and these cuts being passed on to depositors, negative yielding instruments may be an interesting alternative. Also take into consderation the difference between return on capital vs return of capital (the latter being 'guaranteed' by investing in German Bunds for example, similar to investing in US Treasuries for US investors)
From a trading perspective, traders who bought Schatz when it was yielding around -0.50% say 3 months ago have made a decent return with yields currently closer to -0.75%