When you file for bankruptcy, you have to list all of your assets. Not just the majority of your assets, or all of your biggest assets. You have to list everything. If you don't, you could be accused of bankruptcy fraud for concealing them.
People sometimes try to do this in ways they think aren't illegal, but you could still be caught. For example, one businessman had an active person injury lawsuit underway, but it hadn't been finished yet. He decided not to mention it when filing for bankruptcy.
He eventually won the case and got a settlement. A story about it ran in the newspaper, and one of his creditors saw it. Naturally, the creditor was outraged. The man had essentially tried to wipe away all his debt right before coming into a large sum of money, which could have been used to pay back that debt. The creditor reported the issue, and then the courts forced the many to turn over the money from the settlement. It was considered part of his estate, though it hadn't been reported.
People will also do things like falsifying their papers, "giving away" assets to friends and family members, or destroying records to hide what they own. All of these things can cause trouble and bring about fraud accusations.
As noted above, people sometimes commit fraud accidentally, because they simply don't realize that they're not allowed to hide their assets in this manner. That's why it's so important to understand the legal regulations that surround bankruptcy cases. They must be followed, and even innocent mistakes can lead to allegations of fraud.
Source: FIndLaw, "Criminal Consequences Of Concealing Assets in Bankruptcy," accessed Oct. 21, 2016