Erik Plovpenning was not a particular popular king, hence his nickname Erik Ploughpfennig. He put, among other things, a tax on ploughs and that was not at all popular!
As such he faced quite a few disturbances. I.e. people grabbed their swords, axes and what not and gathered to - ahem - voice their protest, often after having beaten up a tax-collector or two.
That happened constantly during the medieval age in DK and usually things settled down after a soothing talk, lots of beer and if necessary an unveiled display of force. And with a bit of luck not much property was burned and no one (really important) was killed.
But not king Erik, he put force behind his words. Sorry, we need the taxes. Pay up or else...
And since it at the time was also mainly free and as such relatively wealthy farmers who used a plough, it was also the free and armed men who constututed the main elements of the king's army - or his opponets - he p*ssed off!
On top of that Erik wasn't particularly gifted diplomatically, so he managed to annoy some of the more important noble families and bishops as well, so it was hardly a surprise when he was assassinated. He was decapitated and his body dumped in a lake - that'll teach him!
In fact the tax for which he is most well-known was a sign that Denmark was becoming a modern country, with taxation to finance a national administration and defense.
It was not such a harsh tax and pretty fair as well. Because practically every free man at the time owned land. The more land and therefore wealth, the more ploughs. Not to mention that this particular tax was hardly a burden on anyone. - But as said before, he wasn't particularly diplomatic so he ended up being overwhelmingly dead...
But there was no turning back, Denmark was becoming more and more feudal and that meant taxes. Those who lived long enough may have remembered that they only had to pay a pfennig per plough under king Erik.