That sounds like a good guess too - testing the boomer's stealth vs the attack sub's listening abilities, and would explain them colliding in the middle of the Atlantic ocean like that.
I thought the sonar operator on the following sub was supposed to listen for the sound of the sub in front turning - but I guess it's hard to be sure and things can happen fast.
Actually both boats are boomers. And both have the mission of not being found - as in running extremely silent. Getting pinged by sonar can make a bit of a divot in a boomer boat commander’s career, since getting pinged by active sonar pretty much indicates the boat has been found. Happened to an Ohio boomer, pinged by an RCN vessel. Anyhow this collision is likely to be a career ender for two captains, if the RN and French Navy policies are anything similar to the USN.