Why does InnoDB table by default uses the UNIQUE constraint instead of the PRIMARY KEY?

Question

In my MySQL database I have a table with 4 columns, the PRIMRAY KEY, two columns with a FOREIGN KEY CONSTRAINT and one UNIQUE index on all fields except for the PRIMARY KEY.

The SHOW CREATE TABLE:

CREATE TABLE `zdb_userbeschikbaarheid` (
  `UserBeschikbaarheid_ID` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `UB_User_ID` int(10) unsigned NOT NULL,
  `UB_PlanDagdeelTaak_ID` int(10) unsigned NOT NULL,
  `UB_Datum` date NOT NULL,
  PRIMARY KEY (`UserBeschikbaarheid_ID`),
  UNIQUE KEY `UQ_UB_User_ID_PlanDagdeelTaak_ID_Datum` (`UB_Datum`,`UB_User_ID`,`UB_PlanDagdeelTaak_ID`),
  KEY `FK_UB_User_ID` (`UB_User_ID`),
  KEY `FK_UB_PlanDagdeelTaak_ID` (`UB_PlanDagdeelTaak_ID`),
  CONSTRAINT `FK_UB_PlanDagdeelTaak_ID` FOREIGN KEY (`UB_PlanDagdeelTaak_ID`) REFERENCES `zdb_plandagdeeltaak` (`PlanDagdeelTaak_ID`) ON DELETE NO ACTION ON UPDATE NO ACTION,
  CONSTRAINT `FK_UB_User_ID` FOREIGN KEY (`UB_User_ID`) REFERENCES `zdb_user` (`User_ID`) ON DELETE NO ACTION ON UPDATE NO ACTION
) ENGINE=InnoDB AUTO_INCREMENT=522 DEFAULT CHARSET=utf8

For some weird reason when I select the full table it's using the UNIQUE index instead of the PRIMARY KEY. An EXPLAIN SELECT * FROM zdb_userbeschikbaarheid; yields:

id  select_type table                   type    possible_keys   
1   SIMPLE      zdb_userbeschikbaarheid index   \N

key                                     key_len ref rows    Extra
UQ_UB_User_ID_PlanDagdeelTaak_ID_Datum  11      \N  415     Using index

It will not use the PRIMARY KEY until I specifically tell it to ORDER BY the primary key.
EXPLAIN SELECT * FROM zdb_userbeschikbaarheid ORDER BY UserBeschikbaarheid_ID;

id  select_type table                   type    possible_keys
1   SIMPLE      zdb_userbeschikbaarheid index   \N

key     key_len ref rows    Extra
PRIMARY 4       \N  415     \N

It just seems odd that the UNIQUE KEY is chosen instead of the PRIMARY KEY.

What's causing this behavior, is it a potential problem for my database? And if possible, how can I change it to use the PRIMARY KEY by default?