If a fund manager leaves the fund they run the case for investing in it needs to be reassessed. This is particularly the case when a manager has run it for a long time and developed a big following, because as well as possible changes in the way the fund is run, a departure can lead to investors withdrawing their assets from it.
Just as in any other profession fund managers change jobs. This could be because they have moved to a rival company to run a similar fund or because they are going to retire. And popular managers with good long-term records may leave to set up their own asset management companies to keep the profits they generate for themselves. Examples include Neil Woodford who set up Woodford Investment Management and Richard Pease who set up Crux Asset Management.
Asset management companies spend significant amounts of time planning for when popular and high-profile managers step back, to try and minimise the amount of money investors take out the funds they run. But this isn't done publicly so investors usually don't know a manager is leaving until a few months before the departure date.