Sometimes it happens. I find a new job or I get laid off (only happened once).
So what do I do at that point? Do I burn the bridge and leave the place in a smoldering heap of ash?
Actually just the contrary. I help even when it's no longer beneficiary to me.
For example, I worked for a non-profit for a number of years. After a while it became necessary for a new employment. Nothing they did, I just had a growing family! I've not been an employee with them for nearly four years now, but I'll still give them advice, do some freelance technical work for them and do what I can to help them.
Another example is my previous employment. Periodically I read something that may benefit them, and I'll send it over to the gent who does SEO, or the CEO. I'll give advice and help to the content writer. I make myself available, even though it does me no good.
Because it's the right thing to do. There are still people there who I count as my friend. I don't want the company to crumble, leaving them unemployed. I want them to succeed. I went to give advice and help that may help elevate not only that company, but their position within.
Basically, I want my people to benefit from my knowledge and skills. I'm giving them a gift of a piece of my brain. It may be a weird, gross brain ... but it can be helpful.
Don't burn bridges when you leave a job. Remember, everyone that's left behind still need you. And you make better long term friendships and business contacts when people know you're a stand up person, willing to help as needed.