"Over the hill" galaxies....see all these old geriatrics wandering around saying, 'I remember, back in the day......!!!!!", and then suddenly a "heavenly host" descends upon them <<<cue angelic choir>>>, hey presto they're young again!!!
Seriously though, infalling gas sounds like the most logical explanation.
If the gas streamers are massive enough, they may be able to get a lens effect from one, providing they have suitable galaxies behind it.