Below the pK, there is little or no charge on the molecule. It tends to bend a lot. This is a snapshot of a vibrating molecule, and bonds are like springs. At any instant, the lengths may be different. This applet is set up to make the average bond length a little greater as the repulsion increases.
Above the pK, the groups are charged and repell each other. This straightens the molecule out.
A real protein has its minimum charge at its isoelectric point. That is the pH where it can curl up or compact the most. At both higher pH and lower pH, it gets increasing charge that creates repulsion to stretch the protein out.