I see you haven't gotten a response in a couple of days so I'll respond with what I know. I've done my share of triathlons, but I'm not an expert on this subject so take that as you will. But...as you may or may not know...the geometry of a TT/Tri bike is different than that of a road bike. The seat post angle on a TT/Tri bike is generally about 4-6 degrees larger than the seat tube angle of road bikes. Consequently, your body is more forward on the TT/Tri bike than on the road bike. As such, that opens up the hip angle (thigh-torso angle) which presumably causes the rider to use their quads more on the TT/Tri bike than on the road bike.

Another possibility: As you're new to riding the TT/Tri, and in the aero position, you may have noticed that handling and maneuvering the bike can be more tenuous since you're not accustomed to riding that bike, in the aero position. That can cause some muscle twitch/tension throughout, but additionally in the quads, which can contribute to the new soreness.

Lastly, maybe you're just stoked about the new bike and so you're mashing harder than usual.

Dan