This move is basically pointless to speculate about, but I'll do it anyway.
There are four possible scenarios. Either both groups are dirty (he would have access to drugs at both programs, so it doesn't matter which one he is part of), neither group is dirty (he would not be missing out on drugs in either group, so this isn't a drug related move), BTC is dirty and NOP isn't (wants to get in on the drug action), or NOP is dirty and BTC isn't (wants to get away from the group before they get caught)
Personally, I think the last scenario is most likely the case. What are the chances that one group has some drugs that the other group doesn't have, especially since they are under the same parent company (Nike). However, I do believe that both groups are doing some quasi illegal stuff, its just that the NOP situation gets more publicity because of their success. All you have to is look no further than Shelby Houlihan to realize what is going on over at BTC. However, I do not believe that any athlete would dope outside of their coaches knowledge, so either the entire group dopes or no one dopes (which makes things a whole lot simpler)
It is possible that Centro has some inside information that the USADA will finally put the gauntlet down on NOP and wants to escape the collateral damage (although it isn't really collateral since he is party of the group). I think that if we knew the current relationship between Centro Sr. and Salazar, then we would know the motive behind the switch. But there definitely is one. Why, after a very good 2018 and a gold medal two years earlier would he leave a group that he's been a part of since college to join a group just around the corner?
Just my two cents, I know a lot of this isn't based on evidence but besides a few newspaper articles, interviews, and a USADA report there isn't much to go on.