It's been said that:
1. Buses run starting at about 6:30 or about an hour before the respective park opens whichever comes first.
2. That early, buses don't run as frequently as they do during park hours.
From that you can deduce that if you show up at the expected starting time e.g. 6:30 it is possible that a bus may have come and gone at say 6:25 and the next bus might not come until say 6:55.
Hence the idea of being at the bus stop before the expected starting time, such as 6:15 for an expected 6:30 first bus. Also it helps to be out a little earlier to reduce the chance enough others may have gotten there sooner to fill up the first bus without you.
Numerous persons have said, so I should believe and assume, that buses stop running from a park an hour after park closing or as soon thereafter as park CMs determine that all guests have left the park.
The hurricane that has been making the news now will not hit WDW but I recall reading a post where a hurricane was bearing down on WDW and a CM went through the Rainforest Cafe at the early closing Animal Kingdom announcing that the last resort buses would be leaving in 20 minutes. Now WDW is not going to leave guests stranded even if the guests were enjoying their meal, let alone lock them outside in a hurricane.