What causes a slight increases in indicated stall speed with altitude?
The U-2C (which the graph is for) had a stall speed of about 79 KEAS, that is 79 knots equivalent airspeed (EAS) at all altitudes. At 50000ft, 79 KEAS is about 80 KIAS (indicated airspeed), but at 75000ft, 79 KEAS is about 83 KIAS. This is simply a consequence of the way that indicated airspeed is measured by pitot-static systems - the measured speed is not quite a true representation of the effect of the airflow upon the airframe. So, the apparent increase in stall speed is just an artefact of the way airspeed is measured, rather than the performance of the airframe itself.  

You can confirm the EAS/IAS conversions for yourself using the Hochwarth Aviation Calculator.