It is technique. Tired riders tend not to stand much (notice Vino on Monday?). During hard efforts and accelerations, he stands for the same reason anyone stands, to utilize body weight to accelerate the bike. And, he's in good enough condition he can do it for loooong stretches, and take brief sit breaks. Even during normal climbs, he's alternating in and out of the saddle, just to rest and alternate muscle sets.
For a really light rider, i.e. almost any climber, there is a lot less difference in effort between riding out of the saddle and seated than for a heavier rider. For a strong, heavy rider like Ulrich, there's little benefit to standing... he's much heavier than Lance, so standing requires supporting a lot more body weight, plus he has more leg strength. He doesn't really need to throw his weight into it. So, all in all, the benefit/penalty trade-off makes standing a bad thing for Jan. That said, I swear I saw him stand on the pedals, albeit briefly, on Saturday.