In any action or proceeding to determine or collect withdrawal liability, if substantially all employers have withdrawn from a plan within a period of 3 plan years, an employer who has withdrawn from such plan during such period shall be presumed to have withdrawn from the plan pursuant to an agreement or arrangement, unless the employer proves otherwise by a preponderance of the evidence.
29 U.S. Code § 1389 - De minimis rule
(a) Reduction of unfunded vested benefits allocable to employer withdrawn from planExcept in the case of a plan amended under subsection (b), the amount of the unfunded vested benefits allocable under section 1391 of this title to an employer who withdraws from a plan shall be reduced by the smaller of—
(b) Amendment of plan for reduction of amount of unfunded vested benefits allocable to employer withdrawn from planA plan may be amended to provide for the reduction of the amount determined under section 1391 of this title by not more than the greater of—
(c) NonapplicabilityThis section does not apply—
(d) Presumption of employer withdrawal from plan pursuant to agreement or arrangement applicable in action or proceeding to determine or collect withdrawal liability