Mr Wilson’s resignation was unusual because, for most of his party and the general public, the announcement came "from out of the blue", and was not, seemingly, prompted by any obvious health issues.
The unexpected nature of his departure gave rise to various conspiracy theories, and suspicion in some quarters that his resignation was forced.
To this day there's a lack of consensus on the "real" explanation for why he left.
A researcher in the field of neurology published work suggesting that Mr Wilson, who had been living with cancer for years before his death in 1995, was actually struggling with Alzheimer's disease while still in office.