Weekly update costs certain contribution points. If your guild’s CP is less than the requirement of former level after update, then it drops.
Guild Level            CP                COST OF UPDATE (CP)
1                           100                               20
2                           940                               188
3                           1550                             310
4                           2300                             460
5                           3400                             680
6                           4000                             800

When we update guild level, the program judges what level your guild is, and then deducts cost of update. According  caculated value to judge which level your guild is after update. For instance, if I have a level 4 guild with 2800 contribution points. To deduct cost of update 460, 2800-460=2340.
The value (2340) is still higher than requirement of level 4 (2300). So my guild will be level 4 after update.