Polyurethane “separating” on table top

When you apply multiple coats of urethane, any dust or small flaws will be amplified with each new coat. If you want the smoothest finish possible, then wait until it is dry, very lightly sand with very fine paper (220 or finer) to remove any dust bumps, wipe it clean with a slightly damp cloth or tact rag, let it dry or buff it dry with a dry cloth, then apply the next coat. There is no reason to use mineral spirits, actually I would advise you not to use mineral spirits.

I see no advantage to applying successive coats with, then against the grain. As long as you apply an even coat with a good quality brush or foam brush and don't overwork it, any brush marks should disappear. The finished product should be so smooth that you couldn't tell which way you brushed the urethane on. Also, if you are using a satin finish urethane, be sure to mix it well, stirred, not shaken. The dulling agents tend to fall to the bottom, so it is important to mix it well. No matter how many coats you apply, it should not become "glossy".

It is very unusual for Urethane to Buckle or alligator skin when applied over urethane. The reason for this type of failure is usually a contaminate on the surface. Grease, oil, wax etc. Did you use the same type of urethane from the same lot? After sanding, did you clean the dust off with a cloth that may have had some contaminates on it? I always use a clean cloth very slightly damp with water, or a tact cloth, never use mineral spirits. Give it a while to dry completely and put on the next coat. The only other problem could have been that the urethane was not mixed well. Always stir, never shake urethane. Don't mix water base urethane with oil base.