Whenever extra weeks are added to the bounty campaign it can mean the following things :

1. The campaign is a success and they want to extend it to more weeks in order to extend the promo of the project

2. ICO's/IEO's is doing great and the marketing team decided to trow another round of coins into the bounty pool

3. They re-calculate the bounty stakes and if they low the rewards they can extend it for a couple of weeks ( this is a shady thing for a bounty to do )

Personally the 1 and 2 are the most usually reasons of why the bounty campaigns get extra weeks and you should enjoy it when it happens.