I'm no war strategist but I'd guess they did it to have them ready to strike Iran if needed. Diego Garcia has been used by UK/US joint operations in the Middle East since the Iraq War, it's not unusual to have American bombers stationed there when the US is on "high alert" or whatever.

To be clear, I'm not saying I support any of this Iran nonsense from Trump. I am very much against him meddling in the ME.