Does it make sense to promote someone one month before they go on mat leave?

Our theory at the end but here is the background.

We are at a big Canadian bank and my director made it clear that our team does not do organic promotions, meaning that even if you are good and you did your 3 years as an associate, you will not be promoted. The only way someone gets promoted on the team is if someone above leaves or if the business grows and there are enough complex accounts to justify a promotion/addition. Our team is the only team on the floor that employ associates while all other teams start at account manager and above. 

There is a lady on the team who announced her pregnancy and submitted her request for mat leave months ago, and a month before she goes on leave, she gets a promotion when no one left, and no new business. Shortly after she left, our team posted the role as a mat leave replacement, but instead of being a fixed term contract, it was a permanent position. Fast forward 18 months, she tries to return back to our team except now there is no head count and she is just a floater under HR. 

So what would be some logical reasons to promote someone right before mat leave, since it would obviously cost the bank more with zero productivity? I asked my director this same question and he just brushed it off by saying "because we can".

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Our theory (amongst the associate class) is that the directors secretly wanted to oust her and force HR to place her in another industry team. The promotion was necessary since HR policy states you are guaranteed a same level position within the department and other groups don't have associate level roles.