When you increase the temperature, vibration in silicon lattice increases which also increases the number of free electrons and holes which gets freed from their respective bonds due to thermal energy.These free electrons and holes contribute to congestion which decreases the mobility and hence more voltage is required to generate inversion layer in MOSFET(Vt increases).
when temperature increases the thermal energy of the electron increases and hence more electron hole pairs are generated... also the electrons have more energy and hence their mobility increases until a certain value.... this leads to decrease in the threshold voltage as there are more minority carriers present making the formation of the channel easier....