"There's a part of me that thinks maybe (Son's) trying to make up for the ... mispricing in SoftBank Corp," he added, saying the pricing of the domestic telecommunications unit's IPO was "way too high."
The buyback comes after SoftBank Group raised 2.35 trillion yen in December by listing about a third of the shares in SoftBank Corp, which on Tuesday reported a 24 percent jump in quarterly operating profit.
Beyond the announced share buyback, Dan Baker, an analyst at Morningstar, pointed to other possible reasons for Thursday's strong gains.
One of those was that SoftBank earnings were presented in a different manner to highlight "sum-of-the-parts valuations rather than discussing results of individual businesses," Baker told CNBC in an email.
Baker also cited the additional details surrounding the performance of SoftBank's Vision Fund and the use of hedging to lock in the company's gains in chipmaker Nvidia as potential catalysts for Thursday's stock move upward.
SoftBank announced on Wednesday that it had dumped its entire stake, worth more than $3 billion, in Nvidia in January.
— Reuters contributed to this report.
Clarification: This article has been updated to clarify that SoftBank shares surged on the Thursday trading day in Japan.