Why is this an issue?

Using operator pairs (=+, =-, or =!) that look like reversed single operators (+=, -= or !=) is confusing. They compile and run but do not produce the same result as their mirrored counterpart.

var target, num = -5, 3

target =- num  // Noncompliant: target = -3. Is that the expected behavior?
target =+ num // Noncompliant: target = 3

This rule raises an issue when =+, =-, or =! are used without any space between the operators and when there is at least one whitespace after.

Replace the operators with a single one if that is the intention

var target, num = -5, 3

target -= num  // target = -8

Or fix the spacing to avoid confusion

var target, num = -5, 3

target = -num  // target = -3