My repairman offered two reasons as to why the wafer-type resistor separated:
1) There was not enough heat when the parts were first attached to the board
2) Heat over time would cause the tin to be consumed leaving only the lead, thus causing separation

Note: In my pictures above, there are 2 wafer-resistors each labeled "510", and there are 3 wafer-resistors each labeled "181." The last digit in the numbers represents the number of zeros in the Ohm rating. Hence, 510 (last digit is 0) means there are no zeros in the Ohm rating, meaning "510" represents a 51 Ohm resistor. Again, 181 (last digit is 1) means there is 1 zero in the Ohm rating, meaning "181" represents a 180 Ohm resistor.