The codes that these items adhere too (ASME VIII, AS1210, PD5500, EN13445 etc..) allow Engineers to design bespoke flanges. These items (particularly girth flanges) are one of the most costly parts, consequently they are designed to minimum diameters and thicknesses allowed by the codes. One of the ways to do this is to use high numbers of small bolts rather than fewer of a larger diameter. ASME VIII/AS1210 are the most conservative and consequently flanges tend to be stiffer but still failures are common.
This is compliant and cost saving however the interaction between adjacent and opposite bolting is extremely high. Every time any bolt is tightened those alongside and opposite loosen.
Many procedures exist to minimise this effect but it is impractical to eliminate it using standard methods.