ESPNcricinfo has learned that the reason for the rule change is that the batch of bails being used during this WPL have been lighting up at the slightest disturbance, even when both spigots have not come off the grooves on top of the stumps. Under the Laws of cricket, a bail is completely dislodged only when the bail comes entirely off the groove.
The BCCI has used the Zing brand of bails both in bilateral and domestic cricket including the IPL and WPL. Each of the Zing bails, which are also used widely in international cricket including ICC events, contains a microprocessor that detects the moment when both spigots have come off their grooves, and causes the bail to light up within 1/1000th of a second. During this WPL, it is understood that the bails began to flash even when one spigot was still resting on its groove. Consequently it was decided that third umpires would base their final ruling on the moment when the bail becomes completely dislodged, with both spigots coming off their grooves.
It is learned that the match officials were told about this rule change on the morning of the Capitals-Mumbai match, which was the second game of the season. The teams, though, were notified on the day after the match.
Source: ESPN Crickinfo