Why are my woofers pumping?

O Holter, the SME V is a very light tone arm. The Atlas/SME combo is fine. 7.5 Hz is a touch low but not low enough to cause trouble. I believe you have said that the woofer pumping is different with different records. 
I'll assume you have not beat your turntable with a sledge hammer. What you are seeing is rumble pressed into the record. The lath used to cut the record is rumbling! I have seen and heard this in a number of so called 180 gm audiophile pressings. Some of these companies are using old hand me down lathes that have been moved around and damaged. If the rumble happens to be around 7.5 Hz your tonearm and cartridge will amplify it. I use a digital subsonic filter. Just for fun I turned it off and played some of those bad pressings and sure enough my subs were flapping around like drunken sea gulls. 

mijostyn

... the SME V is a very light tone arm ...

Not really. The mass of the 9-inch SME V is about 10 g, which while on the low side, is certainly not "very low mass," especially when compared with something like the SME III, which is about 5 g or so.

One more update for all my Audiogon friends. The newly replaced Lyra Atlas cartridge sails through my stress tests without problems. I play Pink Floyd: Meddle, and the woofers react a lot, yes, but they dont over-react. The silent woofer pumping is gone.

My former Atlas, made in 2013, was used successfully over a period of several years, before it broke down, after five years - the needle saying goodbye to the cantilever, for unknown reasons. No mishandling on my side. Although the leaning of the LPs was so-so. My first cartridge became non-optimal, after long use ca 1000 hours, and it remains to be seen if the new Atlas extends the score.  

Good luck o-holter. I do have a soft spot for that cartridge. It is totally unbeatable for things like string quartets and not bad at all for rock.