The academic research suggests that, by and large, buy-backs are...

The academic research suggests that, by and large, buy-backs are good for shareholder returns in subsequent years.
Instituting a buy-back is the opposite of management embarking on an empire building spree through acquisitions.

Management are effectively saying that of all the possible uses of surplus capital available to them, buying the company's own stock looks to be the best use of it
In assessing the prospects of their own stock, management has the advantage of having all the inside information.

Here is an early paper on the abnormal returns following buy-backs.
More recent research suggests that the anomaly still seems to persist to some degree.
https://papers.ssrn.com/sol3/papers.cfm?abstract_id=226564

Cheers