Some high stakes players at both sites noticed that a few of their opponents had win rates that were statistically improbable.
Analysis of their play suggests (conclusively) that these players could see the other players hole cards.
The scandal started at AP and AP stridently denied there was anything wrong until a complete hand history (showing everyone's hole cards) for a tournament was released. It was beyond obvious that the tournament winner could see the hole cards of the other players.
AP finally admitted there was a problem.
UB continues to deny (or not comment) that anything wrong happened.
The player accounts for the suspect UB cheaters was made public, and those accounts have all been closed.
Those accounts had accrued several million dollars in (stolen) winnings prior to being closed.