Problems are various banks and insurance companies who need to put money somewhere. And since there are as many dumb (and vile) fucks in finance industry as in government and since government makes rules where this money can be stored at, those with excess money park it into safest place they can (there is some part of money parked into riskier papers but .gov limits amount of funds one can use in riskier classes). Compared to rest of the crappy states in EU Germany is least worst
hence dumbfucks buy German paper shit as its considered (keyword here as entire modern economy is based on appearance if you believe in something and if enough lemmings believe the same than it works - until that belief is gone ofcourse then it doesn't) least likely to default on their debt. For bonds its basically like this (since there is plenty of demand for them) the higher the likely hood of not being able to deliver principal (100$ in upper case) the higher the coupon (interest) value to attract demand. Since you have rating companies and since you have MSM and big boys (JPM, Bloomberg etc. that brainwash smaller fishes) its quite logical that Spain/Italy/Greece etc.. are more likely to suffer some kind of default event than Germany hence negative coupon for Germany. SUre there is ability to sell bonds and speculate on their future worth but generally due to reserve funds at ECB also being negative (to store money at ECB you have to actually pay them also negative rate) there is basically no totally safe place to store money but Austrian/German bonds. Basically what negative rates for Germany say is this:
Most believe that EU is fucked and that periphery will be fucked pretty soon (10y TOPS) and we better put money into Germany as it will survive or be minimally fucked and we'll either get most of our investment back or later when fuckary starts we might be able to sell this bond at a premium to scared fucks who were fucked by defaults in periphery and will look to exit from periphery into safer paper.
PS: Little is advertised that ECB already owns about 40% (data i think is 12/2018) of all government issued bonds is EU. In other words (ECB is owned by central banks of all EU member states) EUSSR is buying government debt of its member states at an unprecedented rate clearly showing that there is no bid for most of the paper issued (there is data i think on balance sheet for ECB how this debt is structured but i frankly don't give a shit anymore but most likely its PIGS government bonds).