John 5: 18

The root verb used in this verse refers to loosening something unlocking it. the same word used by the Baptist’s referring to his subservience to Jesus while he testified to his unworthiness to “loose” or untie the strap of his sandal. Mark 1:7, which was the task of a slave.

Jesus was continually over and over breaking the Sabbath  recorded in many gospels and this was something they had at this point challenged him on it, “FOR THIS REASON THEREFORE NOT ONLY…” then it gets worse specifically written here by John

This made the Jews more determined than ever to kill Him [to do away with Him]; because He not only was breaking (weakening, violating) the Sabbath, but He actually was speaking of God as being [in a special sense] His own Father, making Himself equal [putting Himself on a level] with God. (AMP)