Welcome to the Strelets Forum. Please feel free to discuss any aspect of 1/72 scale plastic figures, not simply Strelets. If you have any questions about our products then we will answer them here.
Differences in plastic quality and manufacture can cause this problem, basically the plastic looses the chemical that makes it flexible over time, some plastics dont have these particular plasticisers so are unaffected, eventually though all such plastic (with platicisers) will go brittle, this could take hundreds of years or just a few...
generally it seems to be confined to a relatively small percentage of plastic that goes brittle in relatively short time ie say 5-30 years or so, and it will likely be a particular batch... with airfix it seems to be very early 70s or late blue box...
Theres is no way of halting it short of recycling the plastic...