Illustrate music beats
Dyalog APL, 14 bytes
⊖⍉↑'='⍴¨⍨?33⍴6

Explanation
33⍴6 33 repetitions of 6
? random integer in range [1,n] for each of the 33 6s
'='⍴¨⍨ equality symbol repeated each of those number of times
↑ convert list of lists to table of rows
⍉ transpose rows into columns, columns into rows
⊖ flip upside down
Example runs
Input is indented six spaces:
      ⊖⍉↑'='⍴¨⍨?33⍴6
=          ==        =      ==   
= =    =   ==      ====     ==   
= = = ===  ==  === ==== =  ===  =
= = ===== ==== === ==== = ====  =
=== ============== ==== ====== ==
=================================
      ⊖⍉↑'='⍴¨⍨?33⍴6
         =  =  =    =    =       
  =      =  =  ==  == == =  =    
 === == ==  =  === =======  =    
==== == ==  ====== ==========   =
==== ============= ========== = =
=================================
      ⊖⍉↑'='⍴¨⍨?33⍴6
             =    =   =  =       
         =   =    =   = == =     
=      = =   =    ==  = ==== === 
= = =  = =   =    ==  = ==== ====
=====  = == ==  ============ ====
=================================