He had just conquered them, but their loyalty was weak.

When they heard that he had come they cast spells upon the sky, stirred up the clouds, and drove them into most furious storms.
This for some time prevented the Danes from voyaging, and caused their supply of food to fail.

Then, again, the storm suddenly abated, and now they were scorched by the most fervent and burning heat; nor was this plague any easier to bear than the great and violent cold had been.
Thus the mischievous excess in both directions affected their bodies alternately, and injured them by an immoderate increase first of cold and then of heat.

Moreover, dysentery killed most of them.

So the mass of the Danes, being pent in by the dangerous state of the weather, perished of the bodily plague that arose on every side.