This is not uncommon. In most cases it is caused by expansion and contraction of the wood with the change in seasons. When we get those hot humid days in the summer, the air is saturated with moisture. Wood will absorb the moisture in the air causing it to swell or expand. When it starts to get cold outside and you turn on the furnace, the air gets very dry. The moisture will leave the wood causing it to shrink or contract. Because your slate is screwed to the wooden structural frame of the table, this movement of the wood will force the slate to move along with it. As the slate moves back and forth ever so slightly, it can be forced out of alignment.
The only way to prevent this from happening is to keep the humidity level in the room uniform throughout the year. The more extreme the differences in humidity are from season to season, the more the slate will go out of alignment.
The slates can also be knocked out of alignment if the pool table gets picked up or moved.