Can somebody tell me why....'SPOILERS'

I think I have the answer, my friend. If you remember earlier, when the swordsmith had finished Takeda's masterpiece sword, he was showing Lambert a few classic cuts and blocks with it. When Takeda suddenly walked in, he asked if they were using his new sword. The blacksmith lied to cover for Lambert and said no, "we were using this one" (an older sword) But Takeda is no dunce, he strongly suspects, even is sure, Lambert had touched his sword.

To the Samurai of old, his sword was not only a cherished masterpiece and family heirloom, it was, his soul. NOBODY, besides equal or higher ranking members of the Samurai class were allowed to touch his sword (with only his permission, ofcourse.) If a lower rank Samurai, or even an "ashiaru" (foot soldier) handled his sword, it was a grave affrontment. If a farmer or peasant or even worse, a merchant should handle his sword it was considered such an insult that the Samurai might even slay the trespasser for his insolence.

Now, with this being said...imagine if a Gai-jin (foreigner, non-Japanese) handled the sword of a Samurai! This was beyond unforgivable. No Samurai worth his salt would ever handle again such a sword. Although Takeda suspected Lambert had dirtied his sword with his gai-jin hands, he still needed Lambert to draw in the hated Ninja. So he let him go, but was extremely pissed off. This explains his hostile attitide toward Lambert right after this.

So to sum it all up, I believe Takeda reached for the other sword because he believed Lambert had cursed it earlier and maybe thought that such a soiled sword would fail him in this crucial final battle. It's too bad such an old world superstition tripped up such a legendary warrior.