Five witnesses who recounted the massacres to AFP said the Islamist militants feared they would be killed by advancing soldiers or separated from their wives when they fled the town.
They killed the women to prevent them from subsequently marrying soldiers or other so-called non-believers, they added.
"The terrorists said they will not allow their wives to be married to infidels," said Sharifatu Bakura, 39, a mother of three.
Nigeria's military along with forces from neighbouring Cameroon, Chad and Niger have claimed huge victories over the insurgents in recent weeks but defenceless civilians still face serious threats.
- 'Dozens' of corpses -
According to Bakura's account, which was supported by others, Boko Haram fighters received word of a military assault on Bama, formerly an Islamist stronghold in Borno state.
The insurgents had decided to flee to the nearby town of Gwoza before the troops' arrival but first decided "to kill their wives so that nobody will remarry them", she said.
Bukara's husband was killed by the insurgents four months ago but she was spared from a forced marriage because she was visibly pregnant.
Boko Haram forcibly married scores of women in Bama after seizing it in September. Nigeria's military announced the recapture of the town on Monday.
Witnesses who were taken under military protection this week to Borno's capital Maiduguri, 73 kilometres (45 miles) away, said the killing of women began 10 days before Bama was liberated.
The Islamists said "if they kill their wives, they would remain pious until both of them meet again in heaven, where they would re-unite", said Salma Mahmud, another witness.
A vigilante who fought alongside the military in the battle to retake Bama, Abba Kassim, said he saw "dozens of women corpses" in the town.
While other witnesses reportedly a similarly high casualty figure the numbers were impossible to verify.
Fanna Aisami, 52, also in Maiduguri after escaping Bama this week, said the executions followed a warning from Boko Haram's top commander in the town.
"He informed them of the situation and the consequence of the takeover of the town by the advancing troops.
"He warned them that when soldiers killed them they would take their wives back to the society where they would be forced to marry and live with infidels," the mother of seven said, speaking by phone to AFP in Kano.
The commander "said it would be better for them to kill their wives and send them to heaven," Aisami added.
A number of women were shot dead in front of the commander's house, she further said.
Yagana Mairambe, 58, reported similar details but told AFP that "some Boko Haram men refused" and fled with their wives towards neighbouring Yobe state.Nigeria's national security spokesman Mike Omeri told AFP he would try to verify the reports while the military could not immediately be reached for comment.
With Boko Haram gunmen facing heavy military pressure across northeast Nigeria, attacks, including suicide bombings, have persisted, even as the government in Abuja tries to assure voters that March 28 elections will be secure.
The Islamist uprising has claimed more than 13,000 lives since 2009 and President Goodluck Jonathan has faced fierce criticism over failure to contain the violence.
The latest reported atrocities in Bama recall similar massacres at high schools and colleges across northeast Nigeria, where Boko Haram has executed scores of students learning a so-called infidel curriculum.