Why did Hashem harden Pharaoh's heart specifically now, at Makas Shechin?
Ramban #1 and R. Bachye #1: Because, whereas by the earlier Makos, Par'oh's sorcerers encouraged him to stand firm, thereby making it possible to harden his own heart; now in their absence he was on his own and vulnerable, and he did not therefore have the strength to refuse. 1
Seforno: Because boils, which strike the body, are something that is particularly difficult to bear. 3
Why did Hashem begin to harden Pharaoh's heart - a pattern which would continue during the latter five Makos?
As for why Pharaoh was deserving of this removal of free will, see the various answers to question 7:3:1, above.
