An MOS transistor has three leads called a source, a drain and a gate. Its action is almost like an on-off switch. The source is the electricity coming in. The gate is the actual switch, and the drain is the electricity going out. The transistor is either passing or not passing electricity from the source to the drain, depending on the voltage applied to the gate. The applied voltage to the gate is called the threshold voltage. Researchers have discovered that the higher the temperature, the higher the threshold voltage requirements. This is not good, since the transistor may drift out of its design requirements. If the gate does not open when it's supposed to, other components in the circuit are negatively affected.
Every transistor has a time lag, and MOS transistors also have a lag. The higher the temperature, the longer the time lag of the threshold voltage. It takes longer for the gate to open when electricity is applied. Think of it this way: if you are very hot and almost passing out, it will take you a long time to open a gate. This has negative consequences, especially in high-frequency circuits. Components need to have the electricity arrive on time. The time lag could be so severe that other components in the circuit could not even work at all, because of the time lag.
All MOS transistors have internal resistance. Electricity is lost a little bit from the source to the drain as it passes through the gate. The higher the temperature, the greater the internal resistance. This is not good. If too much voltage is lost, the transistor is actually in an "off" state all the time. Other components connected to the drain will mistakenly see an off state, when it fact they should see an on state. The effect is the entire circuit will not work.
The higher the temperature, the lower the lifespan. High temperatures are extremely hard on electronic components. They begin to break down internally. This has a lot of negative implications. Suppose a space probe has to last 10 years, but encounters high temperatures in space. Because the transistor "died" because of high temperatures encountered, the entire probe malfunctions.