As you may know, TEP is an additive used to impart flame retardancy to the PU foam. It's not reactive to the PU components, so that it ends up also acting as a 'plasticizer' (i.e. dead weight that can lower the glass transition temperature). That would be a likely factor why the foam becomes softer (it might even be softening your hard domains in the PU foam). As to why it's less dense -- I'm not certain. It's more dead weight, so in principle that would raise the density. But, less dense means more of the gas generated during the foaming process is able to expand the PU, so if your original formulation (without TEP) was fast reacting/curing, then the TEP may have delayed the curing sufficiently, so that the foam volume could increase further (and decreasing density). Anyways, without more information, all one can do is speculate as to the likely scenario.

Well explained by Kaoru Aou. Additionally, TEP usually has a certain water content of roughly ~0,2%. Depending on the amount of TEP used inside your formulation, the additional water maybe increases the blowing reaction and therefore reduce the density of your cured foam.

Hi, to add what Kaoru says, it could be act as a barrier to gas to be trapped inside the cells like other additives for other foam polymers, so you improve foaming process.

Thanks for your answer @Kaoru Aou