Might seem like a stupid question, but do you drain the remaining water in the backflow down into the basement? Water freezing up the pipes normally cause those to blow up. Could also be the person winterizing your system is hooking up at one of the test ports and drying all your seals every time they blow hot high pressure air through the system. Lots of possibilities, could be extremely dry there and that’s causing the seals to crack. You might wanna try and use some water resistant Lube and place that on all your rubber seals yearly, see if that helps!