I vaguely remember being surprised when it happened, but can't recall whether that was solely because of the suddenness (9 days!) or whether there had been rumblings of discontent from her own party for some time ... I think there was surprise and joy at her finally going cos she'd been PM for all my adult life up till then, winning three General Elections she seemed unstoppable.
Just been
reading this on the Wikipedia; seems she resigned on 22 November 1990, but Labour had been leading the opinion polls since mid-1989, and the widespread opposition to the poll tax, including the March 1990 riot, indicated that she'd lost her touch. Plus inflation was high and interest rates even higher. So I guess the writing was on the wall for some time, whether we realised it or not.