Does this occur only when there are flamers/explosives involved, or does this also occur when those aren't involved?
Could you clarify whether you mean on screen flames and explosions in general or equipped flamers and AT/grenades? I have observed the T-pose on regularly equipped models (e.g. without flamer or panzerschreck upgrades), so I don't think it is necessarily due to squad weapon upgrades or abilities. It's generally very brief, such that the model will switch from a firing pose to the T-pose and then back. Also, models in this pose appear to fire and move along with the rest of the squad.
After testing with the USF, UKF, and Ostheer, I've only seen the T-pose with Infantry Sections (Tommies) in addition to the ones I already mentioned from the Soviets and OKW.
I'll go through my replays to see if I can pinpoint anything else.