Things have changed greatly over the past 50 years in both denominations, and not for the better unfortunately. So, it is near impossible for me personally to be able to say with any confidence what the actual reason is for their fencing the table.

The biblical reason for the practice used to be based upon,
1 Corinthians 11:27-29 (ASV) "27 Wherefore whosoever shall eat the bread or drink the cup of the Lord in an unworthy manner, shall be guilty of the body and the blood of the Lord. 28 But let a man prove himself, and so let him eat of the bread, and drink of the cup. 29 For he that eateth and drinketh, eateth and drinketh judgment unto himself, if he discern not the body."
In short, those who desired to participate in the Lord's Supper must be able to provide a credible profession of faith and not be under biblical discipline from another
recognized church. Intellectually, the individual must have a reasonable understanding of salvation and of the sacrament/ordinance.