You're going to have to look at several things synicus.
Under automation is a button to show log of all actions taken by the game (log all or something), regardless of having all automation turned off it will still alter your workers around if it thinks it necessary.
Do you have new corps?
Upgrading corps will also take hi-tech workers obviously.
You could look for a large shift in population age so a lot of your older people (HTS) retired recently.
You could try to work out if the GM has made one of his sneak changes to number of HTS your corp types use, (This last being the most likely IMO).