In a signed byte the high bit is the sign bit. You can't go higher than 127 for a positive number. You put in 128, that converts to -128.
127 in binary is 0111 1111
128 in binary is 1000 0000
0 in a signed byte is 0000 0000
1 in a signed byte is 0000 0001
-1 is 1111 1111
Think of it like a car speedometer, it was turned back 1 from 0. You look at the high bit, see it is negative, take 2's complement of the rest and it gives you the number. Sounds crazy, but that is how computers store signed numbers.